is there a way for me to check if keys are not used before a giveaway

Apart from trying to activate them yourself I don't think so. And then you're running into an even bigger problem if you start doing that.

Best thing is to make a spreadsheet and plunk all of the keys in there under categories such as "redeemed" and "unredeemed".

No but you can giveaway the key at this group https://steamcommunity.com/groups/dgikinw "Delete giveaway if the key not work" , so in theory there will be less chance that people make problem to you about this (i don't know if in pratice work then...)

Or also at anti-ninjas key drop discussion (but this will NOT give you point\cv) https://www.steamgifts.com/discussion/tnWVh/anti-ninjas-key-drops-no-keys-in-messages-though

It is clearly a conscious decision on the part of Steam to NOT allow this and It is simply maddening.

There are sufficient skilled and talented people that if there were even a basement window slightly ajar via the Steam API to probe a game's title and/or its status (claimed or not), it would have been done. However, should that be (or have been) the case, it is equally certainly that publishing such information publicly would quickly result in Steam closing that little opening in its armor.

As mentioned above, your own personal database (spreadsheet or other) seems to be the best/only way to be sure. But even then there can be issues with keys - while YOU might not have used the key, the source from which you acquired the key might have given you a bad key to begin with for example.