Easiest is the serial number... but there were other changes too.. rear hubs are rivited.. not smooth centers, radious rods on the steering help you determin age, as well as sheet metal setup ( doglegs changed 9n to 2n.. in how they attatch ).. also early 9n had cast aluminum hood.. front steering axle spndles had grease fittings forward on 9n models.. later went to reaward... lotsa little things like that. Basically you hope the engine is oem on that machine, as that is the best indicator of date., then you try to see if the small features I mentioned match upo to the engine searial number date specification.
Other things too.. brake pedals chaged.. steering wheel changed.. type of aux transmission shifter setup changed.. governor chaged to add an oil line.. so therefore, the oil line routing on the engine also changed.. etc.
Hood emblem was changed to add a tiny 2n.. etc.
Soundguy