Is there a way to test steam key?

no already asked it
long time ago it can with somebody how got the game i think

Give it someone who has the game, and let them activate it. If it works, it will say that that person already owns the game, if it doesn't work it will say Invalid CD Key

I think it is possible to check it when you already have the game but I'm not sure.

Steam keys are Schrödinger's cat.

Ask steam support ;)

yup, give it to me and i'll test it :D

It's my key, and I haven't got that game in my Library, so I think It will work. I just don't want to be banned.