They do know it is OEM. Just by the code it generates. Each Serial is unique to either Retail or OEM. Each code is unique to each serial so that way when it generates a code it will tell them by the code that it is either OEM or Retail.
So be prepared for a good excuse. The HDD excuse doesnt always work. Because if you have to format again in a couple of months and use it again then they know something is up.