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 far away and 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.

Network analyzers work at very low power levels. Ours is 20 dBm max (100 mW), with 0 dBm (1 mW) being the normal (default) setting. Even 20 dBm won't damage a conventional probe. However, you should check with you probe vendor before using a network analyzer on a cryoprobe. The issues here would be damage to the internal pre-amp and heating of the cryogenicaly cooled coil resulting in an inaccurate tune.

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.