I have some old steam keys but i am not sure if they are still valid

no there is no way to check:) just drop it as orphan key, if someone is lucky they can get it.

Just make a GA and add a notice that the key may not be working, and the user that wins should let you delete the GA if activation fails

If you plug a key into steamdb's database, it tries to activate it. If the key doesn't work anymore, it lets you know. When the game isn't in its huuuge database, it tells you what it is. Of course, there's a chance that the game isn't already there, and you'd lose the key... but it has never happened to me, and at this point, I'd even be glad to contribute.

You could create a giveaway for them in the group Delete giveaway if key is not working :)

It give a group where the winner must accept the deletion of the GA if the key isn't ok anymore.
https://www.steamgifts.com/discussion/O5LX8/group-delete-giveaway-if-key-is-not-working

Actually you can't enforce anyone to accept that the key is invalid and let you delete the GA. According to the rules, you are responsible of delivering a working key for the game you're giving away.

For your information:
https://www.steamgifts.com/discussion/5fkrA/psa-about-deleting-giveaways

An alternative is to make the giveaway for your whitelist and for the two "delete gib if it doesn't work" groups I know of: Lootboy SG and Delete giveaway if key is not working - anyone can join the groups freely if they agree with their rules, and they have many members.

If you're interested in making giveaways for whatever reason (it's more fair, you want to level up, etc.) the above would be your solution. Members of those groups will accept the deletion of the giveaway if anything goes wrong.

If you're not interested, offer them in one of these groups. The first is "fire and forget", but if you don't scramble the key a bit (I'm sure you've seen examples), bots will harvest them. The second you offer them, people who are interested ask for it, and you choose to who goes what key(s).
https://www.steamgifts.com/discussion/hK2kQ/orphan-keys-drop-thread/search?page=last
https://www.steamgifts.com/discussion/tnWVh/anti-ninjas-key-drops-no-keys-in-messages-though/search?page=last

I would suggest to make a GA if you think they are likely to not have been used. Making the giveaway in a group or as a private GA also might be helpful.
Incase there is an issue, most users would gladly consent to having it deleted.

If you do make a public GA and the winner doesn't want to cooperate...not a big deal. As the Mod above said, they are free to mark it as Not Received. Which is fine, as having not activated or multiple wins is seen as an offense on SG...rather than having your GA marked as not received.

Only a minute percent of users would actually care about that stuff, and that too if the number of such errors are high. Most people don't even care to open user profiles!