I'm pretty sure there is no real, 100% fool proof way of finding out if a guy is bi or not without just coming right out and asking him. I know a few guys who dress well, do their hair, stand up straight, have excellent posture, and people have told me "Oh, he's bi, you can just tell by the way he acts and dresses." And honestly, these are guys who grew up with only their mum and sisters, therefore giving him a more metro-sexual look. Guys are just as entitled to take more concern in their appearance as girls are.
So, in short, there's really no easy way of looking at a guy and deciding that he's gay, bi or straight. There are no visual signs for that. The only way to really get a definitive answer is to ask him out right and hope he doesn't take offense. Because honestly, how would you feel if someone walked up to you and said "Are you bi?" Pretty uncomfortable if you're straight, right?
Hope this helps.