This is copy from an article in Modern Cat Magazine, with information we felt every cat owner should have. Scientists identified facial expressions that are made by cats in pain.

A veterinary research team from the University of Calgary has created a new Feline Grimace Scale for vets, veterinary students, and cat owners alike.

An ongoing issue with the treatment of cats is that pain frequently goes unidentified and therefore unaddressed.

“Cats in particular can be hard to assess for the presence of pain, meaning that they often receive less pain-relieving medication than dogs, even when they undergo similar surgeries or have similar painful diseases,” notes Dr. Daniel Pang, one of researchers.

Use these 3 simple charts of your cat's facial expression to help assess if they may be in pain and require pain meds:

It's a bit technical, but if you want to read the  full report, it's here: https://www.nature.com/articles/s41598-019-55693-8

But for the lay-person, let's just say it was a very detailed behavioral study  conducted in accordance with the Canadian Council on Animal Care guidelines.  They evaluated numerous variables and accounted for observer bias. The results were found to be statistically significant. Quite interesting. I'd like the job of observing cats all day, although not ones in pain. My cats at home can be a big enough pain for me sometimes!