Your baby can communicate from day one when they need to "go"! Sound impossible? Parents know from instinct when their baby is hungry or tired, so why not when they need to go? And if you really think about it, most parents are aware of when their babies are "going", they just don't do anything about it. But some might argue that by the time they realize their baby is going, it's too late. Not so, as I've found out.
Many times when your baby is grunting and you "know" they're going, they're usually just working one up, and thus signaling you that it's potty time. I don't know how many times I've thought I missed because of my baby's grunting, only to find her diaper dry, then she would go right when I put her on the potty. If you start to pay close attention to your baby, you'll notice their signals. Here are some that my babies used:
-mouthing the breast, refusing to latch on, though I know they are hungry
-nursing, then pulling off, then nursing, then pulling off
-sudden fussiness after a period of being really happy
-loud vocalization when in infancy
-face turning red/grunting
-a certain twinkle in their eyes
-becoming really active after a period of laying quietly
-becoming very still and quiet after a period of being active
-fidgeting/not able to stay still/sudden hyperactivity in toddlers
-doing the sign language for poop or potty
-saying poop or potty
Be aware that signals change as your baby grows. But as long as you pay attention, you will learn to read your baby's cues.