> >  How would a prospective purchaser check for this problem?
>
> Look for leaks or evidence of leaks (like whitish/greenish/bluish
> residue) on the bottom side of the heads near the gasket.

The head gasket could leak internally, with little or no external evidence.
One quick-test I usually do is to let the van idle a good long while, long
enough for the temp needle to get above half-way. The temp guage should not
get too much over half way up before you hear the cooling fan kick in.  The
radiator should be hot to the touch.  If the radiator is cool even when the
engine is hot and has been running for a good long time, this is indication
of a coolant flow problem.  This could signal an internal head gasket leak
(but then again, could also be a sign of something less major like air in
the cooling system, a bad water pump, etc.).  I also look closely for signs
of diluted oil (i.e. with coolant mixed into it). A more thorough test is to
take it to a shop that can perform a test to identify whether coolant and
oil are mixing.  As a rule, I tend to plan for the worst, and if a van has
over 100k on it since a head gasket job I would assume that it will soon
need either head gaskets or heads.  Then again, I have found my Vanagons to
be incredibly reliable other than this head gasket problem, so I'm more than
willing to budget $1000 every 100k and take the bad with the good.

- Ron Salmon
 The Bus Depot
 http://www.busdepot.com
 (215) 234-VWVW