If you can get two of these references, you could beat them together in a phase detector and measure them pretty easily.
You could get a really big roll of low loss cable and measure them with a delay line frequeny discriminator, but probably only from 100 hz out. 200' of LMR-400 should do, along with a low phase noise bipolar 1 W amplifier.
There are variations, too. You could multiply the reference up in frequency with a low noise passive multiplier and measure it with a higher noise floor system at either 100 or 1000 MHz.
Of course, some of the fancy new correlation phase noise measurement systems will do it if you have access to one.
Etc, etc.