Here are some photos comparing the fake SanDisk Compact Flash card to a genuine SanDisk card. Most photos can be clicked to view a larger version.
The product number shown on the back of the card is incorrect, showing "SDCFB" instead of "SDCFH".
The fake card also has an extra "TM" logo next to the "CF" logo at the top right of the back of the card, as well as having different patent numbers, and slightly different font for the copyright date.
The serial number is normally printed in small white text on the bottom edge of the card. As the photo above clearly shows, the serial number is missing.
Other people have reported fake SanDisk cards as having labels with lower-quality printing (such as this site).
The fake card came with what appears to be genuine-looking packaging, as shown below.
The genuine SanDisk boxes have a hologram logo sticker on the front of the box, in the bottom left corner, just above the SanDisk logo, but this fake box is missing the sticker.
The "SanDisk Ultra" text on the genuine box is also much more embossed and glossy, and the "II" text to the right of the "Ultra" on the fake box doesn't line up with the rest of the text.
The protective cases for the cards are also different, with the case for the fake card being slightly smaller. The fake card didn't actually fit into its protective case properly, as the case appeared to be slightly too small.
The genuine card had a user guide with the "Ultra II" name on the front, while the fake card came with a generic user guide (which contradicted the box, and mentioned a 5 year warranty).
The fake user guide was just a photocopied piece of paper folded up, while the genuine user guide consisted of several papers stapled together.