The heads should be carefully check to see if they are warped, but the VW
head is quite small and this is not a common problem. What you have to
check are "cracks" and "dents" cause by coolant corrosion, normal wears and
overheating, if those are very small the head can be "regrind" or
"Re-Surface" ("dents" only) at a machine shop. (If dent are very small,
some people use JB weld!!!!) Heads can be check by professional; a
compression test and a leak test can be done.  But like you said, if you
don't have an outside leak, there is good chance that the heads are still
intact. Removing the head is nothing, the only problems are the 2 bolts
holding the exhaust pipe (they will probably be jammed or stuck. If you do
that, pushrod seals should also be change and many other small parts. If
all of that is not possible, you will need new heads.
Good luck, Ben