I bartended for a couple years (and waiting tables was a part of that). it's actually pretty easy. After their food comes out, ask if they need anything else. I

I bartended for a couple years (and waiting tables was a part of that). it's actually pretty easy. After their food comes out, ask if they need anything else. If you see an empty glass, ask if they want a refill. Otherwise, just drift by occasionally and pay attention to the table. If they need you, they'll be looking to make eye contact / flag you down as you walk by and you won't need to say anything if they don't want you. Basically just don't be one of those servers that stands at the wait-station or bar, playing with your phone or socializing with co-workers without paying attention. In my opinion, absent of delivering food, asking if people want refills, or addressing an obvious problem, you really shouldn't be interrupting people. The end result is good service without being annoying.