..is it possible only by running the launcher and seeing which are grayed out? I mean Steam here - I'm pretty sure I bought all of them already, but didn't register all of them.. and I suppose it's not possible to verify which Steam keys are already used (apart from trying them "again").?
Yeah when you run the Steam launcher it will only allow you to select the episodes you have activated and downloaded from Steam, so it's fairly easy to tell from that.
..the point is - being "far from home" and wanting to buy other episodes e.g. in some bundles. Now I know I have all four of them, but please think about it in the future. Even IsThereAnyDeal goes bonkers regarding Cognition: http://isthereanydeal.com/#/search:cognition;/scroll:%23gamelist ;)