On some trains yes (no reservation possible or optional reservation), but on trains with compulsory reservation you need a reservation, in other case you will get off from the train from police or have to pay a normal ticket + fee.

And how can I check which trains require reservations? Because when I am on interrail app, it always says reservation required. 

do NOT use this app-as we have to tell each&every newbee here a few dozen times/day. Best planner, as it covers most of EUR is bahn.com-this will overly clear show (its german after all) IF a train MUST be, CAN be or canNOT be REServed). It will NOT be able to REServe seat only for each country though.

As a rough rule: In ESpana, tops the bill, then ALL TGV=superfast trains in FRance and then ITaly you will have to REServe and shell out for it-plus about all overnight trains. Very often if it is mandatory there are entry checks on the station-which you cannot pass without the proper extra tickets.

thank you. can you also leave the links for train reservation sites for various countries? Bahn.de works for germany only. France, Switzerland, Italy etc.

Here is info about various ways to make reservations.

https://community.eurail.com/train-connections-reservations-47/how-to-get-reservations-105

It would help if you said which trains exactly: origin, destination, date, departure time