Sometimes it's fairly obvious what is wrong with your home appliance -- but how do you find the right replacement part? There is a huge variety of model numbers and appliance types in a single line of products and the parts are usually not exchangeable.
The appliance manual will give you the exact number of your model. The model number in addition to the part number is usually required to get the right part. If you no longer have the manual or the warranty card that came with the appliance, you can usually look on the appliance somewhere inside for a serial number. On washing machines, dryers, and dishwashers, the model and serial number is usually on the inside of the door. Model numbers can also be underneath a machine.
Many parts, especially plastic parts, have serial numbers directly on them. Look at the part thoroughly, as it may be in a discrete place. If the part itself does not have a part number, you may need to reference a diagram within the appliance manual itself.
Here's the most complicated part. Rather than having a picture of each part, most part stores have a small diagram of the part. You will be able to compare your old part to this diagram. In particular, you should measure your part and you should pay attention to scale. If the part number and model is correct and the dimensions all seem to match up, it's very likely to be the part that you need.
There are times when you might encounter a discontinued part. If this situation occurs, you should call the manufacturer directly. It may cost a little more money -- you'll probably have to order it direct-- but you'll be certain to get the actual part you need. The only other option is usually to purchase an appliance that doesn't work but is the same model as your item "for parts."
It can be easier to just call an appliance repair company. Most of them will be more than willing to allow you to purchase your own parts to save money; they'll just give you the right model number and part number so that you can ensure that you get the right item. Then you can simply pay them for the labor, thereby also avoiding making any costly mistakes.