Leaves lose green colour, sometimes with the veins staying green while the tissue between them pales.
Short answer
Which leaves are affected narrows it fast. Old leaves yellowing usually means a mobile nutrient — nitrogen — is short; new leaves yellowing usually means an immobile one, most often iron.
Ordered by urgency, then by how commonly each one turns out to be the answer. The discriminator is the observation that separates this cause from the others — use it before acting.
How to tell: Oldest leaves yellow uniformly and are shed. Nitrate tests at or near zero in a planted tank.
How to tell: New leaves pale or yellow while veins stay green. Red plants lose colour first.
How to tell: Sword plants, cryptocorynes and other heavy root feeders in plain gravel or sand with liquid dosing only.
How to tell: Lower and shaded leaves yellow and are shed while the top of the plant grows normally.