I think most of the aftermarket companies sell temp senders to match their temp gauges. Otherwise, they should at least say what the resistance should be at certain temps.
I usually rig up a bunch of wires and hook up the gauge and sensor out of the car, then drop the sensor in some boilling water. I usually compare the gauge with a different thermometer just to see if its close, or at least reads 100C/212F when the water is at full boil (i'm near sealevel). A camping stove works great for this. It keeps you and your junk off of the kitchen counter.