All tractors have a serial number printed on them. Tractors have serial numbers on them for other individual parts, so one should take care to identify which is for the tractor itself.
If the number cannot be located, it may be that the serial number plate is missing or the stamp is too worn to distinguish the number. If either of these are the case, then it's impossible to know the original serial number.
Knowing the serial number of the tractor is no guarantee of receiving any history of the tractor. The only information it provides is the year in which the tractor was built.