Yes, leave something and show it prominently, not on the seat or they might think you just forgot it. At a sit down restaurant if your bill has not yet been paid perhaps that is a good signal although in a large place the bus people may not know your check status. Leaving a sweater or book at a movie theater works so that no one takes your seat so that is good at a food place too. If it is fast food or upscale fast food I would probably never leave my food on the table. I just pack it up when I am ready to go if I have leftovers and take it with me to the restroom. Even if I go to the refill kiosk at those places I keep my eye on my table if there is a line.
We are victims of our own business where we are reporting on enforcement of the quick table clearance rules.