you can if you go to the store page of the dlc and it will show who of your friends own it

If a giveaway creator would like to check that their winner doesn't already own a DLC, they would first need to add that user into their friends list and then they would be able to see from Steam Store if the winner owns it or not, like cami666 said.

Thanks, no need to be sorry
This website doesn't automatically hide owned DLC-s (which is annoying)