How do you know if someone is a neo-nazi?

There's a lot of fascists but neo-nazis in specific are actual Nazis. So they'll probably be into swastikas and Hitler. They also use a lot of code-words to signal their beliefs. 14/88 is a popular one.

Most modern fascists aren't technically Nazis they're just near enough as to make no difference. With them its about the sorta checklist of fascist beliefs which at this point is borderline mainstream politics in a lot of countries.

Look for obvious or covert racism, intolerance of those not white, ideologies of pure races, etc.

Usually the swastika, stuff like 1488 and Holocaust Denial are dead giveaways.

They look like the lead character from the Matrix but wear brown shirts

They deny the Holocaust and believe in race superiority and stuff

if the price tag is still on their uniform...

They display MAGA banners

Well, if they are pro-Ukraine then .. it's that

Shamrock tattoo