No need to drain the tub, you meter from the end that plugs into the control box.
I'm at home and can't recall the exact values, it depends on the temperature. I want to say that if the water is in the 90's the value may be around 8 or 9 ohms. Swine? If you check both of the thermistors they should read relatively the same value, if one of them is off more than an ohm or so then it is bad.