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
1996
proabably up near the forks of the bike or some where right there
Go to this website: http://www.motoverse.com/tools/vin/yamaha.asp type your serial number in. This will give year and model
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
2008
1993
3jd-05527