If your kids begins playing with his kid, you MUST know if the firearms are being stored properly.
You could take two approaches I guess.

You should be able to figure this out in a round-about way by asking his advice. People love to give advice. Do you own a gun? Mention that you saw his and figured he would be a good person to talk to for advice on making your first purchase. Eventually, you would learn more about his knowledge of gun safety and all. One of the quickest ways to know this is how he presents a handgun to you if you are in his residence. If he pulls one from a safe to show you, does he double check that it is cleared of rounds and hand it to you with the chamber open so you can verify? There are lots of flags like this to be aware of. If you are a gun owner, invite him to a range to build a rapport. The goal being, weather or not you want your child in his house. Too many accidental child shootings happen by curious kids. I would take the steps necessary to feel this out.

Or, if you don't have the time, you can just ask him how he stores his weapons since your kids will be playing together. A responsible gun owner will let you know he is safe by explaining it to you. If he refuses, then that might be your answer.