I don't think you'll be seeing much if you're looking for hallucinations... when I've done it, I just put on some music and chat like I've done nothing. When you start hearing your name being called by the music and when you try to smoke a bowl of non-existent weed with your friend who isn't there... you... well, you still might not even notice you're tripping. >.>

But then when you can't recognize a face because it's all blurry and when you see spider lines and other visual distortions on textured surfaces (Which, by the way, REALLY stand out) then you can tell at least, that your body is heaving up and your peripheral nervous system is getting all out of whack... and when it's out of whack and your body is getting seized up and you keep going to unconcious and back within a snap of a finger, this is when you know you can possibly trip. You just have to be in the right environment and state.