Is there any rule of thumb for when a dog will feel cold?

I know this sounds like a ridiculous question. The only dog I have been responsible for was a German Shepherd who would shake off a sweater when it was clearly below 0 F.

Now I'm fostering a mutt, four years old and about 50 pounds. She's a true mutt – a herding dog's feet, a terrier's head, long legs, shorter hair generally and almost no fur on her belly. Very energetic. She's pretty stoic generally – been with me four months – and hangs in her crate where she has a big, fluffy bed. I'm wondering if there are ways to tell the cold is bothering her other than her refusing to go out. Where I am, zero degree days are unusual at the depth of winter, but not what I'd call rare. Those temperatures are months away, but I'd like to be prepared. Thank you!