Q&A: Has My Milk Come In?
For some women, it's unmistakable — they wake up with a couple of heavy, sore bowling balls on their chest. Other women (especially those with larger breasts to begin with) don't notice much of a change. You'll probably notice a difference in baby's sucking patterns (sucking will likely change to gulping), and baby may seem more satisfied after feeding.
If you don't notice other changes, you'll know for sure that your body has begun producing mature milk when baby begins peeing more, pooping more, and gaining weight.