Both thrash and nu-metal can be good in their own ways. There's no such thing as a "false metal" band. If it's metal, it's metal. Period. Who cares what genre Slipknot is in? Who cares if Testament is from the 80s? They're both metal, so it's not wrong for a metalhead to like them. I personally like both of them, so just stop the stupid debate of which bands are true and which are not.
However, a metalhead may indeed despise such music as rap, pop, or pop punk, for either being mainstream, or full of shit. And most of it is. However, just because a band is mainstream does not specifically mean it sucks. So System of a Down appeared on MTV. Does that mean they're the worst thing to happen to music since 50 Cent? No! They're simply getting their music out to more people. If you really want to vent your anger out because a band is popular, go take it out on their stereotypical fans, for being such airheaded idiots and making the band look bad - not the band itself.
As for the dress code, there is no set one. Most metalheads, however, can be characterized by either shoulder-length or shaven hair, leather jackets, band t-shirts, and lots of studs and spikes in their accessories. There is also no set way to behave in order to be a metalhead. You don't have to do drugs, you don't have to drink, and you don't have to be the toughest S.O.B. in the world who can demolish 20 asses in a fight. You can be a very friendly sweet person who hates fighting and still be a metalhead.
Just be yourself, and listen to all the great metal you love. That, is what makes you a metalhead. Not taking drugs and ranting about true and false metal. That, is what you call an intentional conformist.
False metalhead: (stoned out of his mind) yo u stupid lozer, Slipnot sux dik. Why doncha lizzen to true metal liek Pantera and Testament, u fuckin poser! Get high an' drunk, jus' liek all of uz!
True metalhead: For your information, I like both of them, and I don't need to be a stoned fucktard to show it.
False metalhead: Whuh? U cal meh namez?? Gah! (falls to the floor, knocked out)
Old-school punk kicks ass, but what’s all this emo/screamo/pop-punk shit. * Sigh * Kids these days.
retard trying to capitalise on the image: fred, the 24-year-old wannabee metalhead who thinks korn are heavy, and believes that rap has a place in metal.
NOT metal bands: Korn, Slipknot, Mudvayne, Godsmack, Limp Bizkit, etc