I'm assuming you're thinking about the Spectrum offering, not the old White Box 2-8-0 with it's Reading style boiler and cab... and clicky broken gears...
I look for certain spotting features to determine what version I'm looking at.
First and foremost, the coupler on the tender. The earliest version came with a Rapido in a body mount box, and dummy casting on the pilot.
This one featured the knock-out weight under the smokebox where a small hardwire decoder could be placed.
Later versions would have the Bachmann dummy knuckle coupler in the same mounting box, still with the non-operating casting up front. These may or may not be more DCC compatible.
Finally, the full on DCC model would have their McHenry style operating couplers front and back, and the underside of the tender is perforated for the sound box.
As a WM modeler, I also take note of the road numbers. The earliest release was 763, the second run was 761. There may be another number for the DCC-sound version... I'm not investing in that, so I haven't paid attention.
Anyway, if the item numbers are elusive, you can ferret out the right one with a few context clues.
Lee
