How does a world-jumper get started?

[Flyndaran]

Originally Posted by fchase8

When all the various DC comics worlds merged during Crisis on Infinite Earths, one of the heroes from a world that had disappeared (it was like Batgirl or something) first had an inkling that things were different when her key didn't work in her lock.

To be fair, that was in the eighties, before so much was digital. Another classic way of discovering that things have changed is using cash that have the wrong pictures on them.

Vending machines keep returning your cash, especially coins, because of differing magnetic signatures despite looking "normal".
I found a silver quarter that way. It also sounded different when dropped.

Cash is produced under very strict guidelines. Even small discrepencies will get noticed by someone eventually.