A: Usually you can hear a buzzing or humming sound, from the circuitry. That’s not the operative frequency of the cleaner, just other noise that’s produced incidentally. The surface of the liquid (never run an ultrasonic cleaner without liquid in the tank) will show signs of vibration, jumping around and perhaps generating standing waves.

If you’re still unsure, or want to check how effective or powerful a running ultrasonic is, cut a piece of ordinary kitchen-type aluminum foil, long enough to go corner to corner across the tank diagonally, and wide enough to extend out of the liquid when this piece is placed vertically from corner to corner in the tank. After 20 or 30 seconds or so, take it out and hold it up to the light. It should be punched full of tiny holes. They may not be evenly distributed, especially in less expensive ultrasonics, which will have “hot spots” and dead spots depending on the relative strength of the activity, but you should see at least some holes generally distributed around the foil.

A more powerful ultrasonic might not, after 30 seconds, leave much of the foil, especially if it’s standard-weight and not heavy-duty foil. Some of the more powerful ones can turn a piece of foil into shredded tatters in ten seconds or so. These, actually, aren’t as common in jewelry cleaning, as that powerful an action could easily damage softer metals, especially silver.