How do I tell apart nitrogen, sulphur and iron deficiency just by where the yellowing starts?

Yes, there is a simple field rule based on whether the symptom shows on old leaves or new leaves, which comes from whether a nutrient is mobile or immobile inside the plant. Mobile nutrients can be moved from old leaves to the growing tips, so their shortage shows on the OLD lower leaves first: these include nitrogen, phosphorus, potassium and magnesium. Immobile nutrients cannot be moved, so their shortage shows on the NEW top leaves first: these include sulphur, iron, manganese, zinc, calcium, boron and copper. Now narrow within each group by pattern. Nitrogen: old leaves go uniformly pale yellow, often a V from the tip in cereals. Sulphur: new leaves go uniformly pale yellow-green, common in mustard and pulses. Iron: new leaves show sharp green veins on a yellow leaf, common on high pH soils. So old uniform yellow leans nitrogen, new uniform yellow leans sulphur, and new interveinal yellow with green veins leans iron or manganese. This is only a starting guess. Yellowing also comes from waterlogging, root disease, salinity and pest damage, so please always confirm with your Soil Health Card or a soil or tissue test before spending on fertiliser, and follow your local KVK on doses and do not over-apply.