I am traveling to Salzburg (Salzburg Hbf) from Vienna (Wien Westbahnhof) on the WestBahn train booked on thetrainline.com. While booking, I did not select "reserve seats" option. According to their FAQs, with such tickets, I can sit on any vacant seat (which is not first class).
In such case, when I am looking for a seat, how will I know if that seat is not reserved by any other person?