If you are unsure, then ask one of his friends or someone in his social circle. Try to figure this out before asking him out. If he has a girlfriend, then he is probably going with her.
Be sure to smile and act confident, even if you are extremely nervous. Most guys like girls who are confident and have self-esteem. (Don’t act pompous or cocky, though). Wear something that makes you feel attractive. If you feel beautiful on the inside, then it will show on the outside.
If he already has another date, then don’t ask him to ditch his current date to go with you. It is unfair to the other girl, and will come across as desperate and insensitive.
Share your current plans with him, if you have any. For example, if you and your girlfriends have rented a limousine, then tell him. This will help him make his decision, and show him that you have a fun evening planned.