Occassional flipped shots of Captain Marvel are given away by his lightning bolt emblem pointing left instead of right.
Billy Batson's alter ego of Captain Marvel is a secret according to various episodes, yet Billy and Mentor drive around the country with the Shazam lightning bolt emblazoned on the front of their motor home.
In various episodes of the series, Captain Marvel would address other characters by name even though he hadn't met them previously as Captain Marvel. He had met them previously as Billy Batson.