It depends a bit on the equipment you have, and the desired accuracy.
You say you have an oscilloscope. I would start by wiring the crystal to the scope and giving a sharp tap. The response of the crystal (which has a high Q) will be a decaying waveform, whose frequency you can determine.
To get more accurate you want to apply a voltage and measure the current - that is, the voltage across the crystal and the voltage across a resistor in series with the crystal. The phase relationship between these varies rapidly as you cross the resonance - so even if your frequency steps are too coarse to see the (narrow) resonance you would be able to tell that you passed the point because of the phase shift.
At the exact point of resonance there is a 90 degree phase shift between driving force and response amplitude. This makes it an easy measurement to do accurately - more accurate than measuring the response amplitude.
If you look at the thread posted at this site you will see that measurements of the real impedance of a transducer show one or more characteristic dips - each of which corresponds to a characteristic (resonant) frequency. Of course there is a direct relationship between phase shift and real part of impedance... so if you have a network analyzer, it will give you a more direct determination than the method I sketched above.