It depends. Sometimes its her expression, the sound of her cry, how she kicks her legs and other times im not sure...

If he didn't poo for 3 days, his tummy will be bloated and his appetite is very bad.

I think I keep oberseving his bowl movements. and if the baby is feeling uncomfortable

From experience, I can identify my daughter's reactions..

I can figure that out from my baby's facial expressions

If my child’s face is in pain, I can tell right away

After a while you learn their patterns

When he’s not pooping regularly

You see pain in their expression

Gut feeling? Their expression...