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Here's a good test for an ultrasonic cleaner: fill it with water, turn it on for a while for degassing, and then suspend some common aluminum foil in the tank, without touching the bottom or sides. In short order, you ought to see pinholes develop in the foil; that's an indication that the sonic cleaner is working.

If the pinholes don't develop no matter where you hold the foil in the tank, your sonic cleaner won't be effective thoughout the entire volume of the tank.

Sonic cleaners work better with soap/detergent/solvent than with pure water, due to surface tension considerations. Note that in many cases, you can partially fill the sonic cleaner with water, then suspend a solvent filled second tray or bottle in the tank; the ultrasound will penetrate the bottle or tray and work on your item, but you don't have to muck up your sonic cleaner's tank with the solvent.

BTW, if you're using a sonic cleaner for jewelry, be careful - do NOT use it to clean emeralds, pearls, opals, cloisonne, or anything except very durable metals and stones.