For what is available to most of us is calipers, infrared, or electrical impedance.
Calipers are going to be the most accurate/ consistent in this group. The key is to make sure the same person takes your update tests and uses the SAME sights/locations
Impedance is going to vary quite a bit due the the fact it varies alot upon hydration levels. you can have a swing of up to 5-7% during the day based upon water levels
Infrared uses that scope type deal (keeping it simple) thats presses to the bicep. Inaccurate due the single location of the test.