Easy answer is you can't!!!!!!!
Read the case number (next to breather, NOT fan housing) The number tells you what the case was originally.
In Australia, generally 1800's were AP***, and 2000's were CJ***
Don't forget there were also 1700's and FI motors (mostly 2L) etc so there are more case numbers out there.
Then look at the casting number in the head
http://www.tunacan.net/t4/tour/heads.shtml
http://www.tunacan.net/t4/reference/cylhead.htm
This tells you what the heads were originally. BUT many re-built motors now have AMC replacement heads so won't have this number.
The issue is all motors (and heads) are easily converted to the other, so you can never really know what is inside unless you open it up.
At the end of the day they are all good if well maintained, and power is not hugely different between them. If compression is good, bearing play is within spec and oil pressure is good, just use it :-)
