[mark_in_manc]

If you have a scope, you can measure the resonance. Sit the stylus on a play-out groove, motor turned off. Connect the scope to the headphone out on your amp, or directly to the phono out of the deck if you prefer. Tap the deck / thump the shelf / stamp on the floor.

(A physics interlude - the Dirac delta function, which has value at one point in time only and is zero everywhere else - contains all frequencies. This means it will (potentially) set off all resonances of a system, so long as you apply it in the right place. Since you only want low-freq info, a lo-pass filtered delta function - a woolly thump - will do you fine).

You'll see a (very) LF decaying sine wave on the scope - if it's very well damped, you might not see so many oscillations, and perhaps not even a complete one, but it's worth a go. Find the period T, and F_res is 1/T.

It's unlikely that there are any more-lightly damped electrical filters in the path, compared to the oscillation of the arm on the stylus compliance, but I guess it might be possible. If so, you'll measure them too...

cheers
Mark