I’m not aware of many insider tricks, but there was one guy who would drop a napkin on the floor, and then go to the bathroom, and if it’s replaced and left folded on the table, that’s a good sign, less so if it’s still on the floor or crumpled on the table when you return. He’d always ask for mint tea, and see if it was fresh mint or a tea bag. You can always ask questions about the ingredients that you already know the answer to, to test the knowledge of the servers.
I personally don’t use tricks, as I think it’s unfair to the establishment, for example deliberately dropping a glass of wine, or being a particular difficult customer, to test the service. That stuff should be left to those companies that employ people to go undercover. When I’m reviewing, I try and have as close to a normal experience as a punter that I can, or else you can end up with a false impression.
Just pay attention to everything that’s going on from the moment that you walk in, from the greeting to if they say goodbye, and everything that happens in between, and not just at your table. If it’s the kind of place that sends out greasy plates, or at the high end places that use wide-rim plates, if they haven’t polished the plates so you still see the fingerprints, then you’ll probably notice it in other things.
Attention to detail should be evident