How do I know if this heater is setup for natural gas or propane?

If you can get a good look at the orifice,look at the hole size.Natural has a hole that is usually around the size of the tip of a pen,while lp is smaller.There also should be a number on the orifice.This will tell you the orifice size which you can either compare to the other one(if you know what type of gas it uses).To be sure of the correct orifice,call the manufacturer and ask what size they use for lp and natural,that way there is no guessing.Btw just sniffing to see what has been used doesnt mean the correct gas was used,just trying can have bad results if its set up for nat and you use lp.