Winners tell lies

not sure about anyone there was one time one game i got there was already activated by another account

Lootboy keys are known to often be dupes, so no surprise here. As for your question, steam support used to tell you the date of key activation, but they seem to stop doing that now. Noone will ever tell you the account it was activated on as well.

There is no way to do so (no one can do it - Steam or Lootboy cannot check who activated that key).
You can ask winner to delete GA and write to Lootboy's support and they will add 100 diamonds to your account or provide a new pack for you (they will not replace key)

1. lootboy may occasionally provide invalid keys.
2. regarding Steam keys, if you have the game you tried to transfer in your library, I think it is possible to check if it is an unredeemed one by entering the activation key from the Steam web page.

As for 1, you can contact SUPPORT based on the message content at the stage where you tried 2 and ask them to exchange it for a different key or lot.

If both 1 and 2 are too much trouble, there is a group that agrees to delete the GA if Lootboy's key is invalid.
You may want to take advantage of them.
📝

Steam Group Link
SG Group Link

Enjoy your rite of passage. Unfortunately, that's the con of doing giveaways. I had that happen to me at least 3 times with recent keys from reputable official stores. :/

A couple of times I came across the fact that the keys bought in official stores (for example, Humble Bundle and Fanatical) were activated. This is especially true for very old keys.
So how can you be 100 percent sure of a lootboy key?
PS: In theory, if you are amazingly sociable, you will be able to find out from Steam support the date and time when the key was activated. But you need to be extremely persistent and very talented to do this (I once did this to get a refund from a gamergate, by the way, also an official store). Time and date. They will never be able to tell you more.
However, this is quite complicated and in your case does not make much sense. So just replace the key if you can or ask to delete the giveaway.

That's the risk you take giving away keys you gotten for free (and even on keyreseller sites this can happen)

And even putting a disclaimer up like let me allow to delete this key hold no stake, you could only count on a goodwill of the winner to delete the giveaway.

thanks to all who replied, 60% i got my keys from lootboy and never an error occurred, the key was activated or duplicated, how can i trust the winner..?

Thank you to everyone who has discussed and commented