I've never tried this, but I understand you can have your coolant
analyzed for combustable gases to see if pressure at the head is making
its way into the coolant (as opposed to coolant into the combustion
area, resulting in white smoke at startup). Garages specializing in
radiator work should be familiar with this.

As I understand it there are actually two kinds of head leaks (1. Head
gasket 2) coolant over the top of the internal cylinder barrels, past an
O-ring and large thin metal gasket, into barrels)--the head gasket leak
is far more common. Follow the link below for a description of the two
kinds of leaks.

I'm not sure if both kinds of leaks will commonly result in coolant
pollution. Perhaps other listees know...anyone?

http://vanagon.com/problems/head_leaks.html

Also, here's an explantation of the pressure-test using the pressure
gauge you mentioned (takes place of radiator cap):

http://vanagon.com/info/articles/head_replacement.html

Best of luck,
George
83-1/2 Westy