Checking for chain stretch on an M119 is not really any tougher that any other MB engine. It is a little more difficult to remove the valve covers because of all the plastic parts and if the M119 engine is in a W124 chassis working space is at a premium.
The procedure for checking stretch is in the engine book, You need 4 alignment pins, one for each cam, and you must be able to lock the cams with the alignment tool with the crank positioned at (don't quote me on this I'm doing it from memory) 45 degrees before TDC....
I have a 400E with 120K miles. It has had regular oil changes since new and shows 5-6 degrees of chain stretch. From what I've learned here the V8's with a longer chain and more changes of direction in the chain run are more prone to chain failure than the diesels or gas 4 and 6 cylinder engines.
Good luck - Tim