You can use the Rail Planner App to determine if a particular train or route requires a reservation or not. The Rail Planner 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 Rail Planner App.
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 compulsory”.