AFAIK, palm sensing is not exposed through the graphical touchpad configuration tool. That said, you could try playing with it by using synclient.

To see what options are available, run synclient -l | grep -i palm

PalmDetect              = 0

PalmMinWidth            = 10

PalmMinZ                = 200

Those are the options I see, so, to activate the palm sensing, run synclient PalmDetect=1, and then experiment with the values of the other two.

   "PalmMinWidth" "integer"

          Minimum finger width at which touch is considered a palm.  Prop‐

          erty: "Synaptics Palm Dimensions"



   "PalmMinZ" "integer"

          Minimum  finger  pressure  at  which touch is considered a palm.

          Property: "Synaptics Palm Dimensions"

When you find the right values, create a startup entry in Startup Applications with a command of the following form:

synclient PalmDetect=1 PalmMinWidth=xx PalmMinZ=yy

...where xx and yy are the desired values.