Skin Allergies More Condition_symptoms How Do I Know If My Dog Has Skin Allergies?

How do I know if my dog has skin allergies? - skin allergies more condition_symptoms

He has some fav spots began to itch, and it is a little red. My roommate said he had skin allergies, but I'm not sure what you are looking for. I took a bath last week and the redness disappeared for a day, but then returned.
Thank you for ideas.