You cannot tell by just looking at the engine, since the breaker points cover and assembly is the same between both units as well as the battery ignition setup Wisconsin offered. The difference is all beneath the flywheel.
There is a serial number break between the two magnetos. The Fairbanks Morse went obsolete even during production of the engine, so Wisconsin had to switch to Wico.
Number 6039991 was the engine that started using Wico.
There are different flywheels between the Wico and Fairbanks Morse setup, but I am not for sure if their is any difference on the outside.