First, what Stewart locomotive are you talking about? They are most known for their F units of the late 1980s and early 1990s. But they also made other types of locomotives. Different models had different drive stories.
I know the U25B did have an Athearn drive.
Stewart F units began their production with Kato drives, they then moved to drives with a Bulher motor, at least one batch had a "low torque" motor, which apparently would burn up if they stalled. They fixed that issue with the next run. Then Bowser took over and I don't know about those.
Other than the Kato which is stamped on the bottom of the truck, I don't know how to tell one from another, especially without an original box and paperwork.
Personally I found very little difference in the Kato drive and the Bulher's that I own. But could just be coincidence that I have never stalled them or I didn't get any from the production run with the low torque motors.