You could do with running a leak-down test.
Most good garages (diesel specialists even better) will be able to perform one.
It will test the leakage from compressed air in the cylinders - one-by-one.
You can then determine where the gases are escaping, by listening to air coming from the dip stick, breather, exhaust or even the header tank by means of a leaking head gasket.
The test would give you a reading on each cylinder, and inform you which one(s) have worn rings (probably) and just how badly these are effecting your motor.
Tom