First sign of allergic reaction almost always...
Is a rash. Either mild to severe. But our bodies are amazing; they usually tell us before a more severe reaction occurs.
Mild red marks. Almost like a heat rash will appear on their chest and belly first mostly. Maybe their palms and lips look red too.
I think moms miss this sometimes because it can be so mild and babies get so many skin things as it is.
If you're not sure if its formula or detergent or something else, eliminate everything that could be the cause and just add one thing back. Wait 48 hours then add another thing, wait and repeat c
Watch their skin. It's not fool proof but is a main indicator before it gets worse and then you have an uncomfortable baby with lots of runny poop.