Where budgets allowed, the instrument of choice used to be either the Bio-tek UW-3 (see image below) or the Ohmic
UPM-DT 100AV. Similar test kit was available from
others (BC Biomed USP-100SP, Fluke UW-5, Netech UPM-2000
et al ... all working on the same principle).
This .pdf gives an idea about the current Ohmic range (as does
this link).
Failing that, if money is tight, and you only have just one or two units to check out, you could (do as we once did and ...) make your own "ultrasound output indicator" out of a glass jar (we found a square one - some sort of tank) and fashion a "deflection vane" dangling on a wire frame. The vane to be under degassed water, of course. Trial and error is then required before marking output levels on the glass, as pointed to by the er, pointer. If I remember rightly, we were looking for Watts per square centimetre.
When it comes to frequency, I should imagine that waving a probe (or wire hook) from a frequency counter in front of the transducer should do the trick. We're usually looking for 1 and (or) 3 MHz. Remember not to run the unit for too long without a load (that is, bung some coupling gel on the transducer surface).
Meanwhile,
here is an "learned article".
