Are you asking how to tell what transmission is in it by the serial number or by looking at it?
By serial number, I have no Idea.
By looking at it if it is an HST: The tractor will have a large rubber covered L shaped pedal on the right floorboard, that rocks forward and backward, and has a forward arrow and a backward arrow embossed in the rubber. On you left as you sit in the seat there will be a range lever, that moves between ranges 1-N-2-N-3. and on the fenders there should be in large letters HST. There will be no foot throttle, like a DT will have, and on the HST on your right there will be a small yellow lever high on the fender to actuate the PTO. On the DT there will be larger lever near the center tunnel at your feet to actuate the PTO.
Hopes this helps.
James K0UA