If you have a network analyzer, you are very fortunate. This is the ideal (if somewhat expensive) device for tunning a probe. You want to set it up in single port mode to display the S11 parameter in either linear or log magnitude mode. The choice of linear or log is a preference, but I prefer linear for probe tuning. Set the sweep to cover you nucleus of choice +/- maybe 5 MHz. As you get closer to resonance, you can probably narrow this down to +/- 2 MHz or even narrower for a cryoprobe. The display should show a dip that you want to centre on your resonance frequency and make as as low as possible. The dip should go below about 5 % in linear mode (or ~ 30 dB down in log mode). Usually, you can get it to virtually 0 in lin mode or 50-60 dB down in log mode - unless you have a salty sample :-)
The "gotcha" to this method is that most spectrum analyzers are rather large magnetic devices containing a hard drive. I believe that your magnet is an unshielded 800, so you will need a fairly long cable to connect the analyzer to the probe. You certainly want to keep the analyzer outside the 1 mT line and preferably outside the 0.5 mT line. This can make the display difficult to see while tuning the probe. A solution to this problem is to connect a remote display to the analyzer and have that display closer to the magnet/probe. Most LCD displays will tolerate much higher fields than the
analyzer and are mostly plastic. Our Agilent analyzer has a standard 15 pin display connector, but older analyzers may not. I know people who use this sort of approach with the spectrometer host computer for magic angle setting or probe tuning with "wobb" (qtune) on high field magnets.
I know of one place (Morris Instruments) that has a small portable analyzer that can work near high field magnets. This unit is a sweep generator with a reflection bridge and not a true network analyzer, so it may not be the unit of choice for probe/filter design and construction. However, it is a fraction of the price of an Agilent or Rhode & Schwartz network analyzer, and will do a perfectly good job of routine probe tuning.