Just found a couple of old transducers tucked away in a desk drawer in my
workshop. No clue as to what they're from, make, model, etc, nor as to
operating frequency.

If I recall correctly, the freq is the most important characteristic. Also
seems like the other thing is not to stick 'em on some depth finder that
overpowers them and pops the circuit that fixes the freq. Is all this
correct?

If so, what's some practical, reasonably simple way to determine their
design freq?