How do you find out the year of your motorbike without a vin number?

I bought a Yamaha DT125R which had been used as a field/motor cross bike. The vin number had been removed! Apparently it had been bought at a police auction. Anyway, I think it's a 1999-ish but can't be sure and it's a nightmare with getting parts etc. Does anyone know how to find out what year it is without the vin?