Don't be surprised if they say they didn't find anything, and they don't think its leaking.
Its easy to check for yourself. Don't give them to much grief, you may need them later on.
Like texasscott says, you should be about 3.5 gallons give or take a little.
I fill up to where the gas just enters the neck, within five miles or so of the low fuel light coming on.....does that make sense??
I never fill mine any more then the small ridge at the bottom of the neck, and I do the same thing everytime.
Mines been improving as I've been tinkering. The first was 3.84, and the last was 3.5, and the next one is going to be closer to 3.0. If your in that range around town, you'll be between 35 and 40 mpg.
I set my trip meter, and let it keep adding up miles, and just keep track of how much fuel I buy(the last was about 420 miles, and 11.5 gallons total). I keep a running tally that way. I figure when it gets to 1000 miles, I'll reset the trip meter and start again.