Right click 'My Computer' > Properties. You can't really tell the retail and OEM copies apart, aside from the fact that the retail copy needs to be activated.
except according to M$'s TOS all OEM copies MUST have the details displayed of the OEM manufacturer in <right click / properties>
Right under the registration details at that screen there is a product ID (not to be confused with product key). An OEM version has the ID built in this format: xxxxxx-OEM-xxxxxxx-xxxxxxx.