The number one key to getting the right part for your bike, is to know what bike you have.
This can be very tough because so many bikes are customized and look different than stock.
Titles are not much help either since many states don't state the model or do it differently than the manufacturer.
If you have a Kawasaki or a Yamaha, you can go to those companies websites and punch in your serial number and it will tell you your exact model and year.
If you have a Honda or Suzuki, you can call your dealer and if they know what they are doing they will be able to look up the serial number for you and tell you the year and model.
Either way, right it down. The actual model names are an alphabet soup and even one letter makes a big difference.
In 1982 Honda has a CB750C and a CB750SC, one letter different, TOTALLY different motorcycles with only a few parts in common.
You can find your serial number on the frame behind the headlight on most non sportbikes. Sportbikes vary in serial number location but it is usually on the front of the motorcycles frame somewhere.
Most Ebay motorcycle parts sellers who are parting out bikes should have a serial number you can use to make sure it is the same make and model as yours through the same process you used to determine what kind of motorcycle you own.
Do your homework, it pays.