Directly from DirecTV's HD FAQ's.
"If you open the access card front door panel located on the bottom right corner of your receiver, you'll find a white sticker with the model number. You must have one of the following model numbers to receive DIRECTV HD programming: H20, H21, HR20 or HR21. You can also check your receiver model on your On-Screen Menu: Go to Settings -> Set Up -> Info & Test.?