Hi, Artie.
When identifying these units, the main spotting feature is found underneath the radiator section. The SD50 has four latched doors in six door panels and the SD60 has six latched doors in eight door panels. Needless to say, this can be a difficult process as a locomotive passes at speed so it's best to acquaint yourself with road numbers or have a roster close at hand.
I hope this helps.
Bob
