you find yourself laughing that bit more, feeling brighter than you are, feeling more energised than you have been in a while and possibly also so nervous that you keep running through a list in your head of 'how not to fuck up royally' just to keep yourself calm and focused on anything. She's already a vision and you have probably already complimented her. Staring would be rude, and you can't keep rambling about how great she looks..
and run through the mental list again.. just to have a little lull in the inane chatter that you just seem to be constantly serving up.
Striving to come up with a killer joke, or a killer line that'd catch her attention enough for her to reconsider you as an option.. Desperately wanting to impress, yet trying hard not to appear to be trying hard..
Yeap.. that's how you know you're on a date. =)