Checking legal action

No real way to check yourself, I suppose you can find a trusted mod or someone who owns it to check if it's a valid key.

Not possible. You just have to hope for the best.

Pretty much the only way to be sure it is valid is to find someone who already has it, and get them to attempt to use it.

Course, you'd have to trust them, know them, and actually believe they'll not turn around and give it to someone else before you can trade it.

But seriously, trading keys is dangerous and stupid, very much a bad idea.

I could check for you, if you want to trust me.