One of the most accurate ways to test your refrigerator's thermistors is with a glass of ice water, and I will explain why.
Here I have this thermistor hooked up to my multimeter, and we're going to submerge it into a glass of ice water. If you saw my multimeter before I submerged the thermistor in the ice water, it was reading about 11,000 ohms. This would be a correct reading as the current temperature in the room that I'm in is right around 73°F. It's important anytime you're testing a thermistor that you have a chart telling you what the resistance value of the thermistor should be at any given temperature. I know that this particular thermistor should have a resistance value of about 32,000 ohms at 32°F, and fortunately for us, a glass of ice water is almost exactly 32°F.
Let's do a little fast-forwarding as our thermistor cools off. Here we can see that resistance value climbing towards 32,000 ohms. As the resistance value reaches 32,000 ohms, its reading starts to slow as the thermistor reaches 32°F. So, here we have a working and accurate thermistor. It read correctly at 73°F as well as 32°F. Now, keep in mind, that these resistance values should be within plus or minus 10%. Out of range by more than 10%, plus or minus, it should be replaced.