How would I know if an asset/package I bought in the asset store has already been used in a commercially released project/game?

Questions & Answers
1

If I wanted to release a game commercially or for free. What steps would I have to take to verify that I do not release my game with the same assets that have been commercially released in another game?

2

You can’t. Some games or assets share if it is used or not but other than that you just cant know.

Unless you recognize the asset / behaviour or do reverse engineering.