For an engine to run you need three basics... air, fuel, and spark. Easiest one to check's spark: pull the plugs, connect the leads and ground the bodies to the engine, turn it over and look for spark. If the plugs are wet you're getting gas (but it's not burning for some reason), and it's usually safe to assume you're getting air, unless your filter's way past it's use-by date or squirrels have filled it with acorns.
Mind you, at one point I spent WAY too much money taking my bike to the stealer to find out why it was breaking up at high revs, only to find the end of the elastic strap holding my sheepskin seat cover on was getting sucked into the air box.
Odd things can happen.
