Is there still no way to check if key is valid?

I want to create a few giveaways but I'm unsure which of these keys I already gave to friends (it's been quite awhile). What can I do? I'd hate to create the giveaway and find that a key is invalid that way. I'm like 70% sure these keys are unused. I get "product already owned" message in Steam when I try to use them but it doesn't say if code is good or not. I've checked used keys and it gives me that same message.