Is there a way to safely find out?

Believe me if your dog feels that you and/or her are being threatened she will protect you. My dog is a very friendly guy, never growls, barks, etc. at anyone or anything. One day we were taking our daily walk in the park and a man began to follow us. I didn't think anything of it until the guy started to approach me calling to me and asking me my name. I stopped walking and he kept coming toward us, and my dog growled, showed his teeth and began barking, lunging, and going after this guy. If I wouldn't have had him on a leash, I truly believe he would have attacked this man. Once my dog starting this behavior the guy turned and high tailed it out of the park away from us. I would never want to "test" my dog over something this serious. She will protect you.