I will say that this is a quite important discussion to have about lyrical content, stances, ideologies, affiliations, and allusions of various metal bands; what qualifies a band as NSBM; and the relationship(s) of metal with national socialism, far-right ideologies, and groups and institutions disseminating said ideologies.
My interest in this discussion was spawned from 1) observing and participating in editing activity on the Drudkh page (more on Drudkh later), and 2) a solicitation from @3family6 for my opinion.
Intentional NSBM bands
Labeling bands NSBM is an undertaking that must be used with discretion and have utmost respect for truthful interpretation. I would say if there is reasonable doubt about a band being NSBM, or that a random person in, say, a comments section called the band NSBM once, then it would not inherently be wise to follow suit. If there is little to no ambiguity about the ideologies of the members band or music project, and the band openly writes music in testament to such ideologies, and these ideologies are in philosophical tandem with national socialism and/or other far-right schools, and there is enough reliable sourcing to establish their designation, then the band in question can be safely assessed as NSBM. Bands like Absurd, Thor's Hammer[a], Grand Belial's Key, and Nokturnal Mortum are verified and unambiguous examples of NSBM and can, without any possibility of harming subject interpretation, be considered and pronounced as such. (Side note: Particularly the band Absurd have some gall doing that in, of all places, Germany.) Even if not peddling ideas about keeping a white identity or Aryan bloodline in tact, then simple, superficial, brutish depictions of racism against the non-white "others" can make a band NSBM (Example: the cover of Peste Noire – Split – Peste Noire by Peste Noire features blackface.) To validate any one band being dubbed as NSBM, it would be best practice to use multiple reliable sources making the suggestion or affirmation. Using only one source to affirm a band as NSBM has potential to be problematic, because, depending on the quality of the source, as well as the information expressed therein, there may still be a risk of defaming the band in question.
Ambiguous bands
Where it gets tricky are bands who thrive in gray-area fields, or maybe equip a dog whistle or two in their musical rhetoric. Examples of tactics used by bands toeing the line, but in ways that can still be passable and/or legitimate, may include expressing nationalism for one's country, region, or history thereof (Example: The Varangian Way by Turisas; Forefather); having a militant or military aesthetic, and perhaps even militaristic calls to action toward the listener (Example: Sabaton, Drottnar); writing songs about WWII and/or Nazi history (Example: Slayer's song "Angel of Death".); or having regular Pagan themes and symbols in their music (Example: Heidevolk, Folkearth, basically any Pagan metal). All the examples I gave are music that do not have any NSBM connotations, nor are there any serious pundits or journalist suggesting so. Yet NSBM or RAC bands have used these techniques to the point that such, I believe, are reasonably common among such bands (NSBM or NSBM-like example that combines several previous tropes: Loits).
One suggestion I have for ambiguous, middling bands that are part of the sub-discourse of "are they NSBM or not" is to place sources claiming that they are against sources claiming that they are not (the latter potentially including officially released denials from the band themselves). One technique worth using for ambiguous bands, or bands with past or present affiliations with NSBM characters but without respective lyrical themes, would be to state that the band "have been accused" of NSBM, or use similar phrasing. If a band has at least semi-credible rumors of [possibly] being NSBM, and such have been reported by legitimate sources, but there is still credible uncertainty about the [lack of] far-right lyrical themes in the relevant band's music, then the "accusation" phrasing suffices.
Drudkh
This is a technique I thought of when editing the Drudkh page. After there was some edit warring about whether or not Drudkh could be considered NSBM, I made this edit, which I feel is a reasonable compromise. Drudkh is a band that does not have fully discernible NSBM themes, whose music do not evidently address racial matters, and is not seriously regarded as canonical to the NSBM schools. However, Drudkh have members in common with Hate Forest; they espouse politically-minded Ukrainian nationalism; and, most pertinently, they revere Stepan Bandera in their liner notes of Blood in Our Wells. Bandera is, according to my rudimentary understanding of World War II-era Ukrainian politics, a man so polarizing that opining one way or the other about him is a highly political, possibly even partisan, gesture (to put it lightly). Their second album, Autumn Aurora, has its fourth track titled "Sunwheel", an image that sometimes is made into a swastika. Therefore, these characteristics give pause to some critics and reviewers, historians, or other analyzers of music, to draw the conclusion that Drudkh are NSBM; or, one or more of the band members have committed enough NSBM or NSBM-adjacent activity to be considered toxic. (Also, see this Times of Israel article about the anti-Semitic canard of Jews poisoning wells. Draw your own conclusions.)
Allusions and various interpretations
Generally, if a band's lyrical themes are not consistent with NSBM themes and ideologies, then they are not NSBM. But if that is the case; yet their members have affiliations, past or present, with NSBM characters or circles that have not been disavowed or distanced effectively; or the relevant musicians personally express ideas and philosophies of an extreme far-right nature while promoting their music; then, while not sure-fire, the relevant band or music project is more likely to be validly called NSBM or inferred as parallel to such. As well, if the band makes significant use of dog-whistles or allusions through their music, lyrics, or imagery, yet are not canonically considered NSBM, whether or not they should be dubbed such can be a case-by-case judgement call, but such allusions should be reported on their article(s) for the sake of encyclopedic integrity.
If a band has one or two apparently minor questionable elements of their music or image that are in common with NSBM practice, but the band largely does not address nor seem interested in racial politics or far-right extremism, then the likely safe inference is that they are not NSBM. For example, Moonsorrow is not an NSBM band, and is not seriously considered or reputed as such. However, the S in the band's logo has been criticized as looking like the S's in the Schutzstaffel logo. Intentional? I do not know, but I do know that both are inspired by runes. But Moonsorrow's relationship with Loits (see link in first paragraph of "Ambiguous bands") is worth further consideration, as well as the controversy section on Moonsorrow's Wikipedia article.
J'accuse?
Bear in mind that being accused of racism or Nazism is different from actually being racist and/or a Nazi, and does not mean someone actually is a racist, Nazi, anti-Semite or other far-right type of person. (example: Mirrorthrone, a non-NSBM Swiss band) Therefore, to diminish the influence of a probable solitary character who is simply using "Nazi" or similar terms as a slur against someone they do not like; or someone who resorts to defamation, mis/disinformation, or gossip of uncertain origin to paint someone as far-right; this, again, affirms the need to use multiple reliable sources to state whether or not the band is NSBM, or at least has far-right ties. If multiple reliable sources disagree on whether or not a band is NSBM, make sure to give even coverage as best as possible to the contrasting viewpoints expressed.
Conclusion
I know that my opining is not the final word on which bands to give the NSBM label to and which to not. What I can do is observe which ones are brandishing swastikas, sunwheels, and championing Aryanism while disparaging non-whites, and see that they are a full article of NSBM manifestation. But for such who are not traditionally regarded as NSBM, and are even on prominent labels; but have more sly, perhaps insidious, ways of disseminating what they truly think; I can recognize that, while potentially not enough to earn the label of "NSBM", the band in question certainly has a problematic undercurrent which needs addressing in places such as their Wikipedia articles (alongside those ever-important sources!).
An NSBM band, or anyone who is extreme right, is not such simply because they are proud of their ancestors, traditions, and nation, etc. There is nothing wrong with being proud of such. When it starts becoming xenophobia is when someone prioritizes the mythologized rendition thereof above the lives and needs of "others", the latter of whose mere existences the former view as an intrusive force diluting or obstructing their ways of life. An intrusion to stave off or eliminate altogether.
Thank you for reading, and I hope this is helpful. Let me know of any questions, comments, or rebuttals.
But I apologize if my post was not long or detailed enough.