You will need a seat reservation for most high-speed trains and all night trains in Europe. Reservations are often required in France, Italy and Spain. Trains in these countires are popular and will get full quickly, especially during the summer months.

In many other countries, like Austria, the Benelux and Germany, you can usually get by without reservations. Check the Eurail timetable to find all of your trains, and learn if reservations are required.

You can also check in the “Planner” section of the Eurail Rail Planner app… really useful app for planning your next Interrail 

Different ways to check if Reservation needed or not :) 

Ofcourse the easy way is the Railplanner app of Eurail itself. If you search with different Railplanner like for example Scotty (ÖBB) from Austria or DB Navigator (Germany) you have on the train Details often Details with "Reservation compulsory" or "Globalpriced". Globalpriced means often for Interrail and Eurailers you have to reserve. 

Hallo

Reservations are needed for swiss panoramic trains,as Bernina and glacier Express(during my first IR i payed very much the reservations for these trains,and not all Glacier Express was included:only Rhb in 1990,today is possible to plan the whole trip of Glacier Express St Moritz to Zermatt,paying the reservation only).

For Night Trains generally,reservations are compulsory(i  did many times wien/Budapest-Bucuresti,Praha-Budapest by changing from WLT3 in 2 seat in Budapest kelety)

Talgo international trains Milano/Zh-Barcelona,or Barcelona/Madrid-Paris were interesting (for me ,the regauging at border was very interesting).