Does it say it is oil or gas fired?
Look to see if it has lift out plugs on the boiling and simmering plates.
If there are it has been converted from solid fuel.
Are the oil or gas pipes taken into the cooker through the front burner door?
If they are it is a converted AGA.
Is the AGA front plate of the old standard design, but advertised as oil or gas?
If it is then the AGA is converted.