Over a Gizmodo, Kim Kelly—a journalist with deep knowledge of heavy metal—wrote a terrific guide on how to figure out if the metal band you've just discovered has any Nazis in it.
The tl;dr is that it takes quite a bit of work to orient yourself well in the semoitics of metal Nazis, but once you've spent a bit of time acclimatizing yourself to the main signals, you develop a spider-sense for it. To help you along the journey, Kelly compiled some solid tips, such as:
Square one: the Encyclopedia Metallum. Think of it as a metal Wikipedia, but with much more stringent criteria for inclusion (no 'core besides grindcore allowed!). This long-running and utterly exhaustive directory catalogues thousands upon thousands of bands, living and dead, from dozens of countries; it boasts 144,261 entries and counting, and each band page offers a wealth of information for the curious, from full discography to lyrical themes to band members' past affiliations—all of which is extremely useful for the enterprising sleuth. Want to find out if that rad new Belarusian tech death band you just discovered has any sketchy ties, or figure out how many degrees of separation lie between a spooky Polish black metal band and their local neo-Nazis? The Encyclopedia Metallum holds the answers. Of course, this only works if you already hold a working knowledge of whichever genre you're exploring, and even then, the trail can go cold, but for seasoned metal aficionados, this should always be your first stop.
I realize that not every music fan has necessarily sold their soul for rock'n'roll, sworn themselves to the dark, or followed the left-hand path into the riff-filled land, and becoming adept at identifying fascist metal bands can only be of so much use to the average decent person. However, there are still a few tricks almost anyone can use to figure out if that weird new band your friend just recommended is worth your time. Simply plugging in spicy search terms like "band + controversy" or "band + assault" or "band member + arrest" can be very illuminating indeed, and reading old interviews from before they attracted the limelight is a good way to parse their political views (or see if they have since evolved). Looking at their touring history to find out which festivals they've booked and which bands they're played with can be awfully interesting, too, especially for European bands; at some festivals, it still not uncommon to see an ideological mishmash of anti-fascist, "apolitical," and outright fashy bands all occupying the same bill (like for example when several artists threatened to cancel their 2011 Hellfest appearance until Finnish neo-Nazis Satanic Warmaster were booted off).
Check out the whole piece — it's really interesting. For more of Kelly's writing on the subject, she did a great essay for Esquire last fall about explicitly antifascist metal bands and how they fought against the wave of fascism in 2020.