Trying to see if a key is legit.

If the key has been used already, your friend will still get the "already in account" message.

Assuming the key is for the correct game, it will display as "Product Already Owned" regardless of whether the key has been used or not.

Unfortunately, there's no real way to check a key without activating it on an account that doesn't have the game already, which makes the key no longer work.

Trust is the main thing to insure the key is valid. As others said, your friend will get the default message even if used or not.

Don't trade with randoms if you can't expect them to 100% be telling the truth. If there's a chance that he'll lie to you then you should be looking elsewhere for whatever deal you're trying to do.

When trading keys for expensive games, DO NOT trust what seems too good to be true. Rule of thumbs. Second rule, don't trade with low reps. Make sure they are VERY trustworthy before trading. Third thing to take note of, if you can complete the trade in two parts, that's a big plus.

For example, if the game costs 10 keys, ask if you could send 5 keys, get the game, then pay the rest right away. Not many traders will accept that kind of agreement, mind you, and you would need to have a high rep as well.