I aint never seen one ..... but I do know they DO exist ....
The way I did mine on my Merc E240 was I syphoned off maybe half a jar full of water (When Warm!!) from the expansion tank into a "Pickle Jar" ( or at least a jar with a rubber sealing top) ..... & whizzed off to the local MOT testing station & got him to shuv his Exhaust gas analyiser into the jar!!!!!!..................
YEP Exhaust gasses present ........ HG FOOKED!!!!! ......