I have a York Diamond 80 furance and how can I tell if it was changed to use propane.

May or may not be easy. Check the gas valve, it should tell you if it is NG or propane. If not, remove the pressure regulator cap and see if the small diaphram rod is marked NG OR LPG. If all else fails get the BTU rating, take a jet to the LPG distributor and have them measure it.