There is a serial number on the headset and on the engine cases.
For early bikes this site is a good start.
http://www.dropbears.com/m/models/yamaha/modelcodes.htm
The "vin" number will be on the right side of the steering head tube.
it is a 1991 yz250 b1
under the gas tank
Go to this website: http://www.motoverse.com/tools/vin/yamaha.asp type your serial number in. This will give year and model
proabably up near the forks of the bike or some where right there
1996
You can find the serial number for your bike on the frame of your bike. It is located on the right side between your handlebars and your forks or it will be on the top of your motor on the right side just before your right crankcase or your kickstart.
The 10th place of the vin is the year code. In this case you have a P which is a 1993.
1982 yz 125K
1993
2008
The Yamaha YZ 125 engine with the number 4PE-004634 is from the year 1994. The engine code "4PE" corresponds to that model year, which is part of the Yamaha YZ series. If you need more specific details or verification, checking the vehicle identification number (VIN) or consulting a Yamaha dealer can help.