9 times out of 10,it seems , the shaft seal is leaking. The fix is a new
compressor. The seal can be replaced,but is is so expensive, a new
compressor is best. With a new compressor, you need a new expansion valve
and reciever/dryer.$$$$$$

 A a/c store for service people will have a test kit for 134a or r12. Do a
yahoo search.Lots of online a/c stores.

Also a pressure/tempature relationship exists for all refrigerants.When at
a certin tempature, the pressure is supposed to match , on a pressure/temp
chart.I think this test will only work  when the refrigerant is still in
the tank.When  you are low on freon in the van, the pressure will go down.
It is used to check for cross contamination.

Look for a new reciever dryer in the front left wheel well.  Most change
this when going to 134a. It is only a clue,though. What are your pressures?
Does the system work? If they are over ,say 225psi, it is probably 134a.