How can I tell the difference between the normal spitting up a baby does and something more serious?

This is a difficult question to answer because there are so many possible responses, from “that’s normal” to “hmmm.” The first question is when you mention “milk,” do you mean breast milk? I’ll assume that you do. A couple of things you might check on… breast milk carries antigens found in the food mom is eating. If mom is having cow’s milk products, your baby will most likely have a negative reaction — from colic to vomiting. Other foods that might affect a baby to vomit include cruciferous veggies, beans, and any fermented products. If a food gives a breastfeeding mom gas, the baby will probably experience some discomfort as well. A good test would be for mom to eat only hypo-allergenic foods for a couple of weeks and take note of how the child responds, then introduce foods back in, one by one, and continue observing the response. You should work with a health specialist on this one.

Also, strange to say, some babies are just allergic to mother’s milk. I was, and I’ve seen it before. A goat’s milk based formula can help here.