If your generator is stable enough and can be micro-adjusted in frequency, you might try to find resonant frequency of a crystal. I did that with a rather large batch of crystals and was disappointed in how inaccurately they were marked.
A crystal has two resonances, you hope, and they are very close in frequency. One is parallel and is sensitive to circuit loading. The other is series and is probably a better check.
Many of the crystals I tested had multiple resonances, some dangerously close to marked frequency. They are probably products of sloppy manufacturing but since I never manufactured one I don't really know.