You can look at the fare. If it is Normal fare and you did mot make a reservation, then the tickets are valid for use up untill 2 months after purchase date of the ticket.
Below is what I received information on the ticket I am booking now through this website
Normal Fare Rules:
This train features unreserved seating and is valid for use up until 2 months after purchase date of the ticket. No Changes are allowed to departure or arrival station, class of service, number of passengers. Regional train tickets can not be canceled and no refunds will be issued. All Regional train tickets must be validated prior to boarding the train. Validation machines are at the train station and are yellow in color.