When you have Permanent sandal tan lines on your feet.
When you shop in the nostalgia store by necessity.
When it's Halloween, people comment on your cool hippie costume, and you're not wearing a costume.
When you can't figure out why they're playing Dylan on the oldies station.
If you have two old pairs of Levi Bell Bottoms, that you never wear, hanging in your closet, that you just visit from time to time like a shrine.
When pot is something you have just above your belt.
When acid is something you take Maalox for.
When your hair keeps getting shorter from the top down.
(The guy I got these from wants more for his website, so please add as many as you like!)