The best thing to do is check which coaches on the train are the least busy before boarding (those with the fewest little figures). Then wait at the appropriate place on the platform. This will give you the best chance to find a place for yourself and your luggage.
Suitcase pictograms on the train also indicate luggage racks.
Here’s how to get to the occupancy forecast on SBB.ch:
Search for your connection in the online timetable.
Click on the connection and then on ‘Show train information’.
Find the coach that is most empty and wait for the train to arrive in the appropriate place on the platform. (Please note that the actual train formation is only shown on the day itself.)
Here’s how to get to the occupancy forecast in the SBB Mobile app: Show train information