When trying to identify a motorcycle, you can narrow down the options by knowing the bike's specifications and its general appearance. The YZ250, first built by Yamaha in 1975, still retains a similar appearance for 2011. If the bike is completely unidentifiable by visual means, the Vehicle Identification Number will give you an insight into its heritage.
Look at the general appearance of the bike. Yamaha YZ250s are small, light motocross motorcycles. A motocross or enduro bike will have a skinny light frame, fenders placed high above the wheels compared to other motorcycles, knobbly off-road wheels and an uncomfortable road driving seating position. The 2011 YZ250 bike weighs only 103kg. fully loaded. From at least 2002 and later, YZ250s are reed-valve inducted, an unusual way of motorcycle valve induction, so if you feel like poking around in the engine, have a look. Bikes from 2002 also have disc brakes on the front and rear wheels, but be aware that older YZ250s, such as the 1982 version, have two drum brakes.
Search for Yamaha markings and the YZ250 notation, which is located on the right hand side of the gas tank or on the right-hand fairing just below the handlebars. Sometimes these details can be worn but might still be readable up close.
Look at the colour of the bike. The newer YZ250s, from 2002 to the 2011 models, are painted in the Yamaha team colours of blue and white. If it is a new bike with a different colour, chances are it's not a YZ250.
Compare the bike's appearance to the YZ250's little brother, the YZ125. This smaller bike is similar in appearance but has a smaller engine of 124cc compared to the 249cc of the YZ250 and is trimmer in length and weight than the bigger version.
Find the VIN on the bike, which will be stamped on the frame, and enter it into an online VIN decoder such as Motoverse. The VIN for bikes built from 1982 onward has 17 characters. If the VIN is shorter than this, the bike was built in 1981 or earlier, and calling Yamaha direct, or asking your local bike shop to decode it, is the only way of figuring it out. However, most bikes will be of the 17-character kind, and the online decoder will be able to tell you the manufacturer, year of manufacture, style of bike, description of the bike and engine size range, which will allow you to identify your bike.