How to test a CAN network?

Have a DAF 85 CF euro5 The vehivle will not start and the diagnostic tester will not comunicate with the engine ECU, All connection checked at the ECU and the the cab bulkhead join. What I want to know is how to go about testing the CAN network. I understand its a loop with resistors and should give a certian value but how the hell do you trace for breaks in the wiring.