Answer:
One of the easiest ways is to look at the door windows. Murray doors have a slight arch at the top of the window. Ford and Briggs bodies have windows that are straight across at the top.
Click on the photo below to compare various Model A doors and windows.

There are exceptions. SlantWindshield Fordors made in 1931 had bodies made by Ford, Briggs, and Murray but the bodies were identical and had the arched windows typical of the earlier Murray Fordors.

Most Briggs and Murray bodies had a body tag on the firewall. Through the years, many of these are no longer in place. And also, through the years, a replacement body tag could have been placed by the owner.


Finally, Murray bodies incorporate "caged" nuts where items are attached to the body (like the firewall) whereas Briggs used "D" nuts. Examples can be noticed in the engine compartment in case the body manufacturer's tag is missing or believed to be added.

-- Rick Black, El Paso Texas 1/26/08