Since you're already all paranoid about it anyway, I think you're going to somehow do this regardless of the good advice to let it drop.
That said, since you're gonna end up doing it anyway
..here's how you go about it
..
#1 - you know when he gets off of work, so you need to already be there in your car. Don't park in the parking lot itself
.Park next door if possible.
The trick is to be far enough away and parked casually enough out of sight that he doesn't see you or recognize the car. However it's got to be close enough that you can see him going into and coming out of the restaurant.
It would be really great if you could park in such a way that you can see who the waitress is
..
#2 - be patient. Sit kinda low in the seat and don't do anything to make you stand out. When he leaves, wait 15 minutes or more in case he forgot something, then go in to eat yourself.
#3 - This initial experience is just to get the lay of the place. You don't need to get the same waitress. Just enjoy your meal.
#4 - start doing this every chance you get
..except now you are a regular also
.get friendly with the waitresses because you're a regular. Make up a story that you come to eat there because you just finished work and are on your way to a 2nd job, and this is the best way to get dinner.
#5 - by this time, you'll have figured out which is HIS waitress, and can sit at her station. You can talk about how your boyfriend is upset about you working 2 jobs, and casually ask if she's got a boyfriend.
This sounds like something a crazy person would do, doesn't it?
Yet, I know you've already thought all about this.
Jealousy is a terrible terrible thing. It doesn't keep someone with you, it pushes them away.