Buy a test stick from MB around £36 over here,,or use a length of curtain wire, where when inserted in the tube after removing the seal on the cap, it should be around 5.6cm up the wire with the engine at 80C
I will try the curtain wire method, my biggest problem is in finding the cap !
Dumb I know, is it the black plastic one on top of the bell housing ? which seems to be quite difficult to remove. not tried to much prising yet.