Little help needed

You cant really be 100% sure , even if its used the Already owned thing has priority over dupe so you cant know if it was used or not .

There is no way to know for sure

It may or may not be already activated, but there is no way to know that for sure unfortunately.

This mean, that you have this game. Only person without it can test this key... and use it.

Activate it on other account (if any)

Or ... Here, Orphan Keys Thread