[SOLVED] How can I see if a key is still valid?

Unfortunately that's not entirely true e,g, Fanatical only guarantees their keys being operational for three months. That doesn't mean they won't work longer necessarily but I've witnessed developers going on a rampage and revoking keys years after they're sold on a retail platform legitimitely. In essence Steam is to blame as they empower their sellers to indulge in such craziness without rhyme or reason.

It's still very random even when it comes from official resellers.
I've had very back luck with 4 or 5 giveaways I made from a Fanatical bundle (which admittedly was over 2 years old) but redeemed for myself another 6 keys from the same bundle and all of them worked.

I would say try to make the giveaways in a group that is fine with old keys so that you don't get people using the fact the key is old to say it didn't work to try and sell it or trade it instead.