Hello! I hope you found an answer for your question. The person to ask is the doctor or nurse providing her care, because you want to know that they are assessing pain. Pain in people who are unable to communicate verbally has been studied, and they should know what to tell you to look and listen for as caregivers. They are most familiar with her condition. If you search online, there is all kinds of information on pain and discomfort scales, detecting pain, and communicating. But to ensure that your mom in law is comfortable, I don't think someone online or a web page could answer that as well as her doctor or a nurse that has education and experience with that to give MIL the best care possible. That's their discipline to know what to do.
From my understanding, if they're well trained they know just what to look for during a pain examination-- facial expressions, sounds, breathing, defensive and other behavior-- and they know where how to push, pull and move the body to watch for reactions. Make sure you bring it up with the doctor.