My son has a milk allergy, just like his mommy.
He was going poo 4-6 times a day and is was watery, and not so mushy. He also has a lot of gas and around 5pm he would be crying and in pain for a few hours. He kept wanting to drink his bottle and that would help a bit but then to only compound the ,milk problem.
We placed him on lactose free milk/formula and in 24 hours he was a new baby.
The best way, according to the dr. is to put them on lactose free and if the baby feels better they have an allergy.