to prove he's steeling you'd have to have a record of how much was put in (as airhead said). you'd have to get everyone who shared off the jar to keep a good estimate of the tips as they go in and compare that to the total. it'd also help if you had a night or 2 where the boss wasn't touching the jar and showing a more accurate count.
but as Curval said you'd have to have an accurate count of how much your tip earnings are. and win or lose on this issue, you can bet they will use your evidence against you at tax time, and it will likely cost you much more than the boss is skimming (he knows this and it's why he's not worried).
most waiters and bartenders I've met might claim maybe 20% of their actual tips, I've even seen waiters take a beating from other waiters for giving an honest count of their tips, since the others had audits due to their tips earnings being so different.
not only will you lose more on taxes right now, but if she's worked their awhile (and under reported)the updated tip earnings could be used to adjust back taxes.
I'd probably handle it in a much more unofficial way.
I'd have as many of the co-workers as possible get together in a semi-social setting (ideally off the work property), where the boss is present. having one or 2 large friends, that the boss doesn't know, there as well really helps.
then just have a casual conversation about the tip jar, then immediately just change the subject and go to old work stories, one about a bartender who was caught steeling from the others and how he got his prettythang kicked by some co-workers after work, or something similar.
then another guy brings up the story of how a manager or owner was caught ripping some people off, and how the victim had someone the boss didn't know mess him up. then talk about the best part is how the guy kept working their for years because the boss couldn't prove which of the employees he had ripped off ordered his beating, and was afraid to fire anyone over it, since that would likely make the person behind the previous incident even madder than when he stole from them.
you're just telling old work stories. no direct threats. don't mention directly that you know his hands are getting sticky while he handles the money. it's even better if you can avoid even saying anything directly too him, it's way more effective if he over-hears it.
he knows there isn't much you can legally do about it, and feels secure in stealing from you without risk. so let him wonder if you intend to limit yourself to legal means.
it's wrong (and illegal) to threaten a guy. but it's not your fault if his mind wanders to a place that might lead him to a change of heart, and a new, more honest approach to tips. he might just be one of those guys who needs a little nudge to get his mind wandering down the right path.
