Asking if the restaurant gives free refills on fountain drinks
What restaurant DOESN'T give free refills on soda? How freaking cheap do you have to be to make sure of this beforehand? I can almost understand this question if you're at some incredibly fancy place with artisanal handcrafted avocado soda or some shit, but if you're asking this at Applebee's, you're pretty much guaranteed to be an asshole.
Snapping your fingers
If a customer snaps their fingers at a server, ever, for any reason, it's basically a siren blaring "Hi, I'm an ambulatory tower of raccoon feces and you will want to murder me before my appetizers even arrive." A friend of mine has threatened to break a customer's fingers before, and she's a 6-foot North Dakotan who looks like the Norse goddess Freya and sounds like Bobby's mom from Bobby's World, so you damn well know she'll do it, too.
Touching the server's arm (or anything else)
This one is less "you know they're going to be shitty" and more "you immediately notify your manager to dial 9-1 and then just wait for something to happen before you hit the last 1." Seriously, what kind of sociopath doesn't realize it's not appropriate to touch someone you just met? Particularly someone who you have financial authority over (in the form of tipping). This is something women face far more as servers than men (although it does happen to male servers occasionally), which makes sense, since this horrible shit is endemic to literally every facet of society.