引用自 aDreamingNomad：

How can I check if a Steam code is still valid?

I have steam codes from promotional packs but I would like to offer them to other people.
How can I check it is still valid?

You can't "check" them. You can only try to activate them on your account.

You cant

You don't need to 'check'. Any legitimate store gives you legitimate codes

Don't use illegitimate stores where you 'think' this is a problem. This is like asking if you can be sure if the xbox series x sold by the hobo behind the dumpster at the BestBuy is legit.