9. He always talks about his glory days of using his super powers to save the world, but doesn't have any other signs of Alzheimer's...
8. At least a half dozen times a day, he bemoans the fact that there are no longer any telephone booths.
7. *That* explains why he's always putting his underwear on outside his pants...
6. Every time he sees one of your comics he looks at Grandma, shakes his head and says, "Things might have turned out differently if supervillainesses dressed like that when I was young..."
5. Your grandmother was a spunky girl reporter who got kidnapped a lot.
4. Other kids' grandpas: Pull quarters out of your ear. Your grandpa: Pulls the Flaming Mallet of Hephaestus out of your ear.
3. Other kids' grandpas take them fishing on weekends. Yours takes you to the Phantom Zone to rescue kidnapped scientists.
2. Other kids' grandfathers start their war stories with "One time at Fort Dix..." Your grandfather always starts his with "It all started when FDR called us to the Justice Battalion headquarters."
...and the Number 1 Sign Your Grandfather was a Golden Age Superhero...
1. His DependsTM undergarments are color-coordinated with his cape and boots.