Any way to test codes for things you own?

Nope. You've got to just write careful notes.

Besides, you can't give away individual bundle keys as front-page giveaways, so if you want to give it out here, post it on the forum and we'll find out soon enough if it's used or not.

You can give it out, but now in the form of a giveaway since you are not supposed to give out individual bundle games as giveaways.

There is no way to check keys aside from adding them to your account.
If its used, you'll get a duplicate key message. If not it'll add to your account.
Though if you know someone who already has it that you trust you can have them check it, basically if its unused it'll prompt that person to install the game.
Or at least it should.
Edited for better wording.

Give it to me. I'll check! :D

That explains why those 2 games from the 2nd Be Mine I tried to gift keep on getting knocked down. Alright, here's the Eufloria key. I'm like 25% sure I traded it for something cool (I know I traded away Space Chem at some point), but if you get it, let me know.

GBFMI-24LZL-QNAYY