To see all the available DLC for a game you already own, without having to go looking for specific names, you can select 'My Games Apps' from the home screen, locate the title you want, press the Menu button and then select 'See in Xbox Store'. Scroll right and you'll see all the currently available add-ons for that game.
If you've bought a season pass, redeemed a code or have already purchased the content, you'll see an install button at the top of the list of options. Alternatively, you can hover over a game in 'My Games & Apps' and press the Menu button, then choose 'Show all add-ons'.
