use your googles.... There are several good appliance diagnostic web sites.
Also, if it is a newer washer, it probably has some internal diagnostics. There will be some oddball handwaving to make it spit out a code. (It will be something silly like: turn on, push these 3 buttons in order, and repeat 2 times.)
It will spit out a code that will really narrow it down. If you have the manual, this may be in there... and there is often a tech manual hidden inside the machine. (I.e, open it up and it will be in a plastic sleeve inside.)