I always like to see that the engine is stone cold and that the van has been
sitting in one spot for a while when I go to check out a van.  If the heads
are leaking decently then you will see one drop of coolant hanging from the
lowest spot on the bottom of the engine under the head (a little ear that
sticks down on each side).  If the engine is warm sometimes this tell-tale
coolant will be dried up.  If you are really picky have them remove the lower
engine tin that cover the pushrod tubes.  Take a look at the gaskets and see
if you can see any cracking or seepage.
Hope this helps,

Ken Wilford
http://www.vanagain.com
John 3:16
Office (856)-765-1583
Shop (856)-327-0027
Fax   (856)-327-2242