Fever detection in cats?

How do I tell if my cat has a fever? Without a thermometer? She gave birth to one stillborn kitten a few days ago and seemed fine after. She just climbed onto my lap and when I scratched her ears I noticed they seemed very warm... Is this anything to be concerned about? I'm hoping she doesn't have an infection or something from her pregnancy. She gave birth about 2 weeks after her due date if that makes any difference