How can I know which Magicka DLC I have?

Right click on the game and click on "View Downloadable Content"! :D

Right click it on Steam, then click "View Downloadable Content".

Edit: Ninja'd.

Go here and then click on every DLC. You own the ones that have "Play Now".

So those are those that we already have or are those that we're missing?!?!!? :D
I think the game shows i dont have grimnir and idk what else...

Right click the game in your library -> Properties -> DLC

There you see all DLC you own.

Thanks a lot for the info everyone.