On old cast iron heads you could do a magnetic particle inspection. On these aluminum heads you have to liquid penetrant inspect them to really determine if there are any cracks. But, for the most part the pressure test is run at a pressure high enough that if you don't leak in a 4 hour period, the head is likely fine. Running pressure tests at operating temperature and operating pressure is probably dangerous - I never asked why but the shops I have used don't do it. And for aluminum heads I ask to have them Penetrant Inspected if I am looking for cracks.

Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)