How can I check if the winner doesn't own a DLC?

Cant u see the DLC in his/her games without adding?

For most DLC, it is unfortunately impossible to check for ownership without adding the other person.

Add the person on Steam, check the storepage of the DLC then it should (or hopefully not) show up is unfortunately the only way

Giving away DLCs is a bad idea.
First winner out.

yep dlcs and removed steam games are hard to check. sadly most people wait to win before checking if they do actually own it