How to tell?

You can't without activating them unless G2A tells you what they gave you.

Activate.

If you get "already owned" error, you will see the name.

If you don't have the game already, it will get activated.

There is NO WAY to get name from the cd-key without trying to activate it firstly. If G2A didn't tell you that, you won't know. However, you can ask anybody with higher number of games to try those keys for you, as there is high chance that he owns all of the games already, so you'll get the names (but if he doesn't, obviously, you will lose that key as it gets activated).

i read G2A

;_;

"""""""""""random keys""""""""""""
enjoy your games that was free on ultracheap bundles :)))