How do you know when your milk has come in, when breastfeeding?

Tweet

OK So i have had four kids and never breastfed (bad, i know!). I want to breastfeed number 5, but I am worried about milk supply. During the first day or two, do you have milk? How do you know? I don't remember feeling engorged until post partum day 4. Does that mean I didn't have any milk before that? What do you do if you don't have milk right away, what do they eat? Thanks in advance!