Is there any way to know if a game has been free before?

Not an official list, but here you go:
https://www.steamgifts.com/discussion/pJRbR/can-we-have-a-list-of-games-that-cannot-be-created-as-giveaway-unofficial-list-inside

If you try to give a game away and can't find it on the list, that means it had been free.

search discussions, names usually show up.

For 100% sure, open https://www.steamgifts.com/giveaways/new and type the games title.
Sadly you do this manually.

Maybe in the feature there will be script for this.
Idk if SG have public API for list of games (+ free games).
Will be helpful if so.