There is no hard and fast way to diagnose MSPI in an infant. Usually, a number of factors are looked at together. They include
Deterioration in behaviour
Green and mucuosy stool
Blood in stool
A doctor might order a blood test to measure the levels of eosinophiles, a type of cell that is present in high numbers during allergic-type events. In addition albumin levels can indicate whether or not the infant is having difficulty absorbing nutrients. This becomes a serious problem when the condition is left unchecked and the intestines become severely irritated.
Many physicians might wait for the more severe symptoms (weight loss, bleeding) before recommending that any action be taken. In my opinion, there is no reason not to try the simple elimination of dairy products for two weeks to see if anything changes.
In addition, if you think your child may be reacting to something you are eating, keep a food diary. Record in detail what you eat, and when. Also record your observations about your child. See if you can find any correlations.