A few points from my own experience.
If your hands are hot and sweaty, dry them before handling any gun - even the "plastic" ones have steel parts that rust. Some folks have "poison" hands that are especially damaging to steel; if you are one of those, be extra careful. When you are done looking, don't wipe down the gun with your handkerchief or on your clothing; return the gun to the clerk who will (should) have a special cloth for the purpose.
If you have gotten an OK to try the trigger pull, try to check it while stopping the hammer from dropping. If you want to dry fire, ask about that specifically.
Do not field strip the gun unless there is a good reason and you have obtained permission to do so. And unless you know how. If you break anything, be ready and willing to pay for the damage.
When done looking, check the gun again, and hand it to the clerk. Don't leave it on the counter where someone else could grab it and run. The clerk entrusted YOU with the store's property, don't betray that trust.
Again, DO NOT point the gun at anyone, especially me; I hate the screaming when I break someone's arm.