The five classic symptoms are: Rough idle, excessive white smoke (especially on cold start), a persistent knock in a specific cylinder, increased fuel consumption, and finally, oil dilution.
The Details:
Common Rail injectors are electrically controlled solenoids. When they start to fail:
Internal Leakage: An injector stuck slightly open will dump raw fuel into the cylinder. This fuel washes the oil off the cylinder walls (scuffing) and eventually mixes with the engine oil. Check your oil dipstick: If the oil level is rising or smells strongly of diesel, you have an injector leak (oil dilution).
Return Flow: A failing injector often allows too much fuel to return to the tank via the return line rather than firing into the cylinder. This causes hard starting and low rail pressure faults.
Knocking: If an injector is stuck open, it will fire at the wrong time, causing a distinct mechanical "marbles in a can" sound.