Updated: Oct 23
Trace a line backwards with the tip of your tongue, starting at the back of your top teeth. First you'll feel the ridged bone that your teeth protrude from. Further back and up, you'll find the roof of your mouth (or hard palate). Further back still, the hard palate runs out and you'll feel soft tissue instead. This is your soft palate, and with a bit of practice, you can raise and lower it at will.
You can also watch this video here where I film the inside of my mouth to demonstrate the soft palate in action.