Is it possible for a doctor to tell if a person has herpes by just looking at the affected area?

Generally yes: There are 2 common types of herpes simplex infections. Type 1 is orolabial and manifested by 'cold sores' where the facial skin meets the lip border as well as oral mucosal ulcerations. Lately, type 1 is increasingly implicated as the cause of genital herpes however. Type 2 is classically known as genital herpes and manifests by crops of painful skin lesions on or around the genital region.

Yes: The appearance can be typical and diagnostic.