Perhaps a "poor mans" way to do this might be: mount a thin circular disk under or over one of your saw bands pulleys (or on the motor pulley)which ever is easier to do. Have a bunch of "Holes" drilled around the circumference of the disk, at some kind of close interval.
Have a "Prox" switch or other detector that will cycle on/off depending on if a hole is over it or not. It won't matter which, you will see why.
then bring in that input.........
In a Brain, have that input going to an AND block, and the other leg of the AND block is looking at your Spindle CW LED, then the output of the AND block,
will go to a NON-accumulating timer (you set this time that is the I'm Jammed, value). The output of that goes to the "Stop" or "E-Stop" do button as a terminus.
scott