First and easiest test to clean them is to twist it back and forth a bunch.

Just spin it left/right a bunch, then reset.
Do this for Pan, Gain, whatever dials may be affecting the signal.

If 'dirty' is heard or noticed a spray cleaner may be in order, along with some more twisting back and forth.
But I'd start with twisting each dial back and forth a few times and then returning to original position, and checking.. Have the speakers/headphones on at the time so you can hear if one of them is getting dirty (you'll hear it when you turn it).

Make your sound your own!
.. I do not work here, but the benefits are still awesome