look for obvious leaks, and blue/gray smoke out the exhaust. The biggest thing with diesels is oil pressure/quality. these engines are built heavy and run low rpm's. which means as long as they have have good pressure and don't burn oil ( a sign of cylinder wall/piston ring wear, valve steam/seal wear, oil bad turbo bearings.
One thing i recomend doing is pulling the turbo intake hose the (largest pipe coming from the air cleaner) off and spin the turbo by hand, it should spin easy, and have only a slight side to side play if any.
If it's the original turbo and it's in good shape then chances are the rest of the engine is too.