Sadly, psychology has not yet advanced to the point where we can give people electric shocks for thinking things

So long as one could convince someone psychology (or magic) has advanced to the point where we can give people electric shocks for thinking things, one could possibly trick people into monitoring their thoughts themselves.

"So, let's test this out. Practice thinking of a jar of pennies and we'll see if it triggers the shock." "OK." bzzzzzzz. "Ow, that hurt!" "Brace yourself next time. It helps. Try now. Brace, but don't think of the pennies." "OK." "Now think of the pennies." bzzzzzzzz "@#$%! You're right, bracing helped a lot" "Good. A few more tests to make sure it works, then you go wear it around all day."

The subject then braces every time he thinks of the pennies, which the monitor detects.

Neither deception nor electric shocks are really necessary, of course. People very often change what mental associations they have without them.