Is there anyway to check if the winner has any dlc?

Add him on Steam and check the store pages of the individual DLCs. If the winner has them, he/she will show up in the "x friends already own this game" field.

if you put in GA rule that win is delivered directly through steam, you can require winner to be added as friend [and than be able to check the DLC''s]. If he/she refuses to do so, it is ground for reroll

When you have him on Steam as a friend and you "fake buy" the game/dlc Steam will show if you click the option sent to a friend directly on Steam...
Edit: So if you take the Collectors Bundle Steam will show you this way which DLC of the bundle the winner owns.