A D820 and D830 are very similar. The hinge cover which says D830 will fit on a D820 making it hard to tell the difference. After buying a "D830" which turned out to be a D820 I spotted some more fakes.

Here are a few ways to tell:
The service tag. This is located on the bottom, and in the BIOS. Go to Dell's site and you can get the original system configuration.
The D830 has a round S-video port on the back. There will also be a round power jack. A D820 has a third USB jack where the S-Viewo jack is on a D830
The D820 has an IRDA port on the left side between the WiFi switch and the headphone jack. This port looks like a shiny black rectangle. This is missing from the D830.

Either one is a nice laptop, just make sure you are getting what you paid for.