How do I know if my milk has come in?

Engorgement. You will know that your milk is in if your breasts are feeling very engorged. You may even notice leaking of milk out of your breasts without any stimulation. Try to feed your baby every 2-3 hours. This will help to alleviate your engorgement.
If baby gains weight. In short - you can't know exactly how much baby gets - you just can't measure it. But the whole process is pretty simple. Breastfeeding in supplying nutrients and materials to grow and gain weight as well as fluids to keep hydrated and function. So if baby is pooping and peeing, chances are the body has enough materials and fluids that it can waste some. Also - if baby gains weight, your fine.