I want to check the validity of the key

There's no way to check, except try to use it. There are groups that have rules that if the key is not working, the winner agrees to delete the GA.

The Steam DB extension https://steamdb.info/extension/ will show if the key has already been activated. https://store.steampowered.com/account/registerkey But you have to try to activate it first.

You can try and giveaway the keys in this https://www.steamgifts.com/group/OZCha/dgikinw or this https://www.steamgifts.com/group/aaQSf/lootboysg SG group.
There is no guarantee users will follow group rules, however.

If a game was decent or I cared for CV and I and would expect 80+% chance, that it's working, I would use one of the above mentioned groups. Those group rules are usually adhered to, cause breaking them means an instant kick.
Otherwise, if the game was decent, I would use the "anti ninjas" thread and for trash the "orphan key".
Cheers :)

Thanks for the lots of comments
If I'm concerned, I'll use a specific group.

By the way,
ようこそSGへ ^^