This remedy is used whenever acidity (sourness) is present. Most often it will be useful for babies suffering from colic.
Whenever stools are strongly acidic and sour smelling you can think consider this remedy (although compare Mag mur which also has the sour discharges).
It can be important to compare Nat phos with Mag mur and Rheum both of which also have sourness as a key feature. Rheum is used specifically for diarrhoea in teething infants. Mag mur suffers may also have this. The key differentiator that will lead you to Nat phos is strong acidity in a sufferer.
Everything is Sour
Abdomen bloated
Yellow tongue
Colic
Indigestion