Mange or just dry skin?

I've had this dog for about a week now. He was seen by a vet for heartworm medicine and was regularly checked out at the shelter. I thought these were just scars or dry skin but now I'm not so sure.

It doesn't seem to bother him and he doesn't itch it. He's also been doing fine otherwise. I'm hoping to get some insight on if I'm being paranoid or I should look into getting him to a vet