Any way to check Steam S/Ns?

Hey,

I've got a bunch of Steam S/Ns for games I already have sitting around my computer, but I don't know if they're spares, or if I've already used them. My question here is, then, is there any way to "check" a S/N to make sure it hasn't been used, so I'm not giving away used S/Ns in giveaways? I've never seen anything like this, and was wondering if anybody else might have.

Thanks.