How to tell if a bird is in Pain?

I realize that quakers are lactose intolerant however, I was wondering how would we be able to tell if they were in pain?... or does it work differently on birds?

I know when humans are lactose intolerant, they would get cramps and use the washroom very frequently. Would the QP's feel the same?

Has anyone seen their quaker in pain?.... and what were the signs?

Thanks