The formula for figuring out if a band is "punk" is very simple.
Major label = not punk. The two are completely 100% mutually exclusive.
Having performed any song or songs with the use of orchestral backing = not punk (the Ramones get a buy on this one because it was done at gun point, Phil Spector is a whack job...)
Other criteria can also be used to further whittle down your contendors but it's sort of like buying a Rolls Royce, if you have to ask...
No other genre is shot through with posers more than punk which is sad since punk is more about pure attitude than any other genre, making punk posers some of the saddest losers on the planet.
Punk was/is about the refusal to accept the mainstream, it's about doing it yourself regardless of lack of technical prowess. It's about picking up a guitar with a fistful of rage and writing music despite how many people tell you that you can't. It's not about huge record sales or tons of fans, it's about the experience. Punks were the true outlaws of the music scene, seeing hack pukes like Green Day marketed as punk makes me sick to my fucking stomach.