If you're not sure if you're playing with this guy, don't worry -- he'll tell you. Only vegans talk more about their lifestyle choices than pothead gamers. Within the first few sentences he speaks, he'll find a way to mention how much weed he has just smoked, what type of weed, the device which he used to smoke it, and his future plans to smoke more. Then he'll "forget" he told you he was stoned and mention it again every five minutes. If you still don't get it, he will loudly take bong rips directly into his microphone until somebody, hopefully, finally gives him the high-five for which he's apparently been left hanging all of his life.
This is a common tragedy that occurs when people mistake a hobby for a personality -- you'll find some version of it in anybody that's way too into anything. The bike girl won't shut up about gear ratios, the fisherman turns every conversation back to trout, and god help you if you meet someone into CrossFit. The only problem is, the pothead gamer is trying to merge two mutually exclusive passions: one that obliterates his reaction time and judgment and one that relies entirely on reaction time and judgment. He will loudly proclaim, without prompt, that he plays better high. But this will seem at odds with the amount of time he spends lost in hallways and staring at his own feet. But it's all OK: He'll make up for the fact that you have to carry him by bringing the music.
By which I mean he will blast shitty rap into his headset.