How accurate a read are you looking for? A cat can be used to give a very, very general idea of temperature.
The colder the temperature, the more a cat curls around itself to sleep. If your cat’s lollin’ around, stretched out on her side or back, you can bet it’s warm.
If said cat is tightly curled, with face buried and tail wrapped as tightly around her body as possible, she’s cold. A sweater might help, but cats don’t like them very much. And they look silly in human clothes.
In other words, get yourself a sweatshirt.