Select a game that has challenges from within the app (you don't actually need to own the game). It might take a few minutes because not all Xbox One games have challenges. At the bottom left, under where you sort achievements, you'll see view # challenges. Click that. If there are a ton of challenges (like Shadow of Mordor has), you can click "Filter challenges" followed by "Running Now." If no challenges are currently running, move on to the next game. If there are just view it for the achievement.