How do I determine when she is really in pain?

My grandmother is living in my home . She is 85 and in the severe stages of Alzheimer's. Her latest habitual behavior is to say "Ow" or "Ouch" every and any time I do anything for her. Whether it's changing her clothes, walking to the restroom or dining room, or even just putting her glasses on in the morning, she says these two things. Sometimes changing her clothes for bed at night she starts crying like a baby and yelling to her long deceased mother that I am being too rough (which obviously I am not). I have taken her to the doc and called the ambulance a few times for false alarms and nothing is wrong with her. Is there any "magical" way to be able to tell when this is just because of her regression and when she is truly in pain?