Help Please with these spray arms

I have a brand new Kitchenaid KDFE104DSS3 dishwasher that I bought from Lowes for "Parts Only"It was and is brand new but had been dissassembled for some reason.The bottom spray arm was missing so I bought one and put the machine back together.It fills with water and seems to be draining properly but I have learned that the spray arms are not spinning.Someone told me that the motor might be bad but before I buy one Id like some advise from you guys.There is a little white motor attached to the sump assembly and I dont know what it does.There is also another bigger motor that I beleive is a pump motor.Any way which one of these motors would be the culprit.Any advise is appreciated.