You can never be 100% sure but for 99,99% of the time there will be no problem to get a seat on trains where reservations are not mandatory. 

DB and SBB, and maybe also other train operators, have a system for showing what demand that is expected for a certain train. If you check this a few weeks before travelling and you see that demand will be high, then it can ve wise to make a reservation. 

For Switzerland reservations are a waste of money even when demand is high. Trains run very frequently.

For many regional trains reservations are impossible. 

Hi Daryl - if you want to be sure of a seat then get a reservation.  If you don’t get a reservation then you can’t be sure of a seat (as per your original question).  That’s the way it works.

However, as Annab says above you can be pretty sure of a seat on a lot of trains.  We were on a couple over the weekend around Hamburg and Bremen.  We didn’t reserve anything because it was last minute.  The train from Hamburg to Bremen had a massive demand in 2nd class.  There were hoards on the platform waiting for the train.  If there were any unreserved seats left you would be fighting 50 other people to get to it.  In 1st there was high occupancy but there were still seats available.  On the way back the train was mostly empty in both classes and we had no problem finding seats.

So - if you want to be sure of a seat, reserve one.  If you don’t want to reserve a seat then accept there’s a chance you won’t get one.  Obviously the timing (Tuesday mid-morning vs Friday afternoon) will impact availability.