Excessive pressure can be due to a head gasket leak, it can also be due to overheating. There are kits you can use to test for the presence of combustion products in the coolant, but they may or may not be reliable.

Does the upper radiator hose still have pressure on it in the morning when the engine is cold? Assuming the cooling system is tight and not leaking, that's usually a pretty definitive test for combustion products in the cooling system.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]