I have found, that the most simple way of hanlding the task of measuring the activity of jaw muscles is quite simple. You know if you shut your ears you can feel the inner vibrations of things like talking or even eating.

If you buy ear plugs, I would suggest silicone ones.
This provides you with opportunity to hear, given
that your hearing is quite good,
to feel the vibrations such as
heart beat at times, but most noticable,
is the activity of the muscles
because they are close to the ears and of course produce contractions that resonate.
I have noticed few times that while looking or feeling the muscle itself does not yield any results, this method, should give you the ability to "hear" or feel if your muscles are twitching.

I would be interested to know of electromyogram of some sort can measure the nerve activity of the muscle via sensors of the skin. They do these kind of tests at the chiropractic offices for the back to see if muscles are in spasm.