You can use the Discover EU Travel App to determine if a particular train or route requires a reservation or not. The Discover EU Travel app is a free download for iPhone and Android. On your mobile, go to the App Store (for iPhone) or Google Play (for Android) and search for Discover EU Travel App. You will also use this app to travel with your pass.
Simply use the Trip Planner function to look up the journey you plan to take. If a reservation is required, you will see an [R] reservation symbol and the comment “Reservation required”.