Jitter Measurement Equipment
Two oscillators (Identical, and ideally at least somewhat electronically tunable), phase lock them at 90 degrees, feed both to a mixer and then lowpass the result, look at the spectrum with a low frequency spectrum analyser or even a PC soundcard.... Simple and cheap. 

This is the quadrature method, which works well for close in noise that the notch filter method usually does not do so well for as it by its very nature notches out the carrier and usually a few KHz or so around it. 

The notch filter method is basic spectrum analysis, but with a notch filter tuned to the fundamental because the SAs usually only have 100dB or so of dynamic range and that is not really sufficient to see phase noise at more then a few tens of kHz out if your osc is any good. I like stub lines made out of bits of LDF450 for building high Q notch filters in this sort of application.

If you want commercial kit, keysight have a phase noise measurement set that is excellent, but more of a hire then a buy proposition unless you are feeling really flush.