That's a tricky one.
If you could access the exhaust ports you might be able to tell - 'maybe' -
see further below. 
Or pull spark plugs and smell in there for a-frz smell or by the spark plug
color.
Here's what might work- - leak down test.
However, you might only have 100 psi in a leak down test, and exhaust
pressure is many times higher than that of course, and might not show up in
a leak down test.
If it was dual exhaust you could sniff the two tailpipes with a smog
machine, but it's not dual exhaust so that's a problem, IF coolant is also
getting into the exhaust. 
I was laying under an SVX engine mounted in a vanagon once, looking up into
the 6 exhaust ports and noticed one darker than the rest.
That's the cylinder  that had the problem.  Though you'd think coolant
wouldn't be getting into the combustion chamber and going out the exhaust
especially. .

The answer is, that I can think of so far is 'there may not be a way to tell
for sure prior to disassembly.'
If you are trying to do just one side only, come up with some way to pick
one head, and do that one first and see what you get.  Of course it's likely
that if only one side is doing it, the other side maybe nearly ready to
start doing it also. 
Scott
www.turbovans.com 

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
John Mosley
Sent: Saturday, December 01, 2007 3:21 AM
To: vanagon@GERRY.VANAGON.COM
Subject: engine pushing exhaust into coolant system

What is the best way to find out which cylinder is leaking exhaust into the
coolant system?
           thanks
                          jack  91 GL
                                 84 wolfsburg