OP, just ask the person behind the counter. They'll appreciate you asking, trust me on that.
I work at a gun shop. Sadly, I'm used to people casually putting their finger on the trigger and pointing the gun at me as soon as I clear it and hand it to them. I'm also used to customers pulling out their own guns, claiming they're unloaded, and when I insist on checking it myself I find a round in the chamber. Heck, most gun store employees have even had more than one instance of a customer who pulls out their own loaded gun and absentmindedly points it at us. It's happened to me several times only in the last year. I even had a coworker who had a customer pull out a Glock and try to show him a problem with the rail-mounted flashlight by casually pointing the gun right at my co-worker's face with a finger on the trigger. When my co-worker finally got the gun away from the customer, he found it was fully loaded with a round in the chamber.
OP, what I'm trying to say is that you're way ahead of the curve simply because you're already thinking about proper gun store etiquette. But now that you ask, here are some basic rules we'd love to have more customers follow after we hand them a gun:
1) Don't point the gun at anyone.
2) Keep your finger off the trigger unless you're aimed in.
3) Don't aim the gun at anything until you ask which direction they want customers to point the gun (at our shop we want all customers pointing at the small targets at the top of the wall behind the counter).
4) Ask before dry-firing the gun.
5) Ask before disassembling the gun.
6) Don't slam the slide home on a semi-auto; ride it home gently.
7) Don't flick your wrist to close the cylinder on a revolver; close it gently with your other hand.
8) Don't talk about breaking laws.
That's all I can think of at the moment. But like I said, you're WAY ahead simply because you're actually thinking about proper etiquette. Just relax and ask the salesperson any questions you might have about proper etiquette; he or she will be pleasantly surprised you actually asked.
0331: "Accuracy by volume."