If you suspect a dodgy injector, with the engine idling slacken off the fuel pipe to each injector one after the other.
If the tone of the engine changes when you have slackened pipe off, that injector is OK.
retighten the fuel line before doing the next one.
If the engine tone doesnt change when you have slackened off an injector then this is the faulty one.
Dont undo the fuel pipe all the way just enough to allow the diesel to come out. Bear in mind it is high pressure !! so wear safety specs.
wizard