Unsure if steam key is usable?

I'd say create a private GA with some trustworthy people and ask them politely to let you delete it, should the key not work.

There used to be a "Show CD key" option for games installed in your library but i think they removed it "except some games", So the best think to do in your case is what "Smollo05" said.

The date should be most obvious, Steam shows you when you got the game and if that matches with when you bought the bundle, then it might require more inspection.

You can also click on "Create Gift Link" instead of showing the key. If it creates a gift link, then you're probably good to go. If you already redeemed the key, it won't create a gift link but directly show you a key instead.

I'd remember when that button "Show Key" (Or something else) appeared intact after used. It was a long time ago, so i think your key isn't available anymore. I think they changed something and all buttons went through a reset.

Check out the Ninja Retreat group.