I always find that the BEST thing to do with a dog when you first meet them is TALK to them - yes, talk.

Stop 5-10 ft from them and talk. At that point, I find (at least with ME) one of 2 things usually happens. The dog stops (and possibly sits), and gives me the "head cocked to the side" look, or the dog approches carefully. I just keep talking in a crooning voice, and depending on their reaction, leave my hand out at my side palm UP, or I'll crouch a bit, and put out my hand palm up, LOWER than his head. From there it's usually only a few minutes till the dog is getting ear scritchies, and the owner is shaking their head going "I don't know how you did that"

Of course, there are the dogs, you look at how they respond to the talking and you say "I think I'll pass on the petting - best leave this one alone" You do have to learn to read THEM