As above really. Obviously Glow plugs are hidden away, so whats the process?

If you can't use the glow plugs or injector hole, it is possible to do it electronically with some diagnostic equipment ,basically it measures cranking resistance

on individual cylinders :-a low resistance would indicate low compression. 

As Patrol man said if you can't remove the glow plugs or injectors you can use an oscilloscope with a pressure transducer.

Pretty sure I've done one before via the glowplugs on that engine. Had lots of rags all over the engine bay/bonnet/windscreen but wasn't as messy as expected

Pretty sure I've done one before via the glowplugs on that engine. Had lots of rags all over the engine bay/bonnet/windscreen but wasn't as messy as expected

 

Thats probably why I found used engine oil under the lip of the seal by the wiper cover then. Previous owner told me that it had been compression tested, and I thought using a mechanical one would be messy.

 

They went to all that trouble and all they needed to do was replace the injector seals and injector clamp bolts lol

 

 

Thanks chaps.