Your check would determine whether or not the sending unit is going bad. If the coolant is easily full, your sending unit might need replacement. It is not difficult, and pretty cheap IIRC.
Keep in mind though those things have a pretty low threshold, and flash the "idiot lights" easily. In other words, what may look like full to you may be considered too low by the ECU.
Also Stan, the days of mixing coolant with water are over. Most coolants come pre 50/50 mixed with distilled water from the factory. When you are coolant shopping, be sure to closely read the ingredients on each bottle you will be surprised. For some reason the $8 Prestone has the exact same ingredients as the $3 "Car And Driver" brand..
Old: '96 ZJ 4.0, 32" MT/R's, 4.5" Lift, Mickey Thompsons, JKS Disco's
New: '02 WJ Limited 4.7L, Stock for now