What are signs my breast milk is coming in? 

There are a few things that you need to know about how your breast milk comes in and what the signs are.
First, you need to understand the process of lactogenesis. This is the process that your body undergoes in order to produce breast milk for your baby.
Lactogenesis I: starts 16-20 weeks of pregnancy, ends around day 3 postpartum
Lactogenesis II: starts around day 3 postpartum and ends about day 10
Lactogenesis III: starts around day 10 postpartum and continues until you decide to stop nursing your baby
Your milk comes in around 70 hours postpartum or day 3 for most women.
There are reasons for delays such as c-section, diabetes, etc
Your body will tell you that your milk is in:
breast fullness
nipple flattening
mild increase in body temperature
achiness
Abnormal signs:
true fever
flu like symptoms
We talk about reverse pressure softening to help your baby to latch on to your engorged breast.
Signs from baby's perspective that your milk is coming in:
gulping milk
full off of one breast each feeding
pulls back or unlatches with letdown
you may hear a clicking sound as baby regulates milk flow
Treatment for engorgement (if you are uncomfortable):
express milk from full breast
schedule ibuprofen
ice/cold compresses for 10 minutes or so after feedings