The simplest way to find out the resonant frequency of a XTAL (or "crystal" or "rock") is to oscillate it. What if you do not have (yet) a simple crystal tester circuit and a frequency counter (or an HF receiver)?
I was in a hurry and I could not assemble a simple oscillator. Then I turned around and saw the MFJ-259 antenna analyzer... would it work?
I picked a known frequency XTAL and held it at the antenna socket: SWR and Z were as if I had connected nothing. Then I swept the frequency close to the stamped one and I could clearly see the impedance dip while passing over. I could actually see two distinct dips, probably series and parallel resonance. Nice!
It is important to look for the dip on the SWR analog meter, not the display or impedance meter. The frequency knob has to be rotated VERY slowly and remember that the MFJ-259 VFO is pretty unstable. Rotate so slowly that, for an unknown rock, you would probably have enough time to heat up the soldering iron and throw together a simple oscillator!