First and foremost is just common gun safety:
1. Treat it as if its loaded. Always.
2. Keep your finger off the trigger until ready to shoot.
3. Don't point a handgun at anything you don't want to shoot.
4. Know your target and what's behind it.
Some of these are tricky when handing a firearm in a gunstore, but don't be flippant.
It will almost certainly be unloaded. Check anyway. As far as dry-firing; it's fine on most modern handguns, but until it's your handgun, be sure to ask first. Also, when racking the slide (for a semiauto), don't let it slam home on an empty chamber. Be sure to hand it back grip first, and with an open action.
If you have a certain model in mind, someone might be able to give you something specific to look for in a used handgun. Have fun, and be safe.