There are tons of fake Memory Sticks out there, and while some of them do work, many feature less than reliable performance. mavy, a programmer from PSX Reality, has come up with a comprehensive, homebrew method to check if your flash card is a fake. The test won't be fast, and it will involve doing a reformat of the disk, but it does appear to be quite comprehensive.
However, I've also heard that there's a poor man's way of checking for the real deal. Go to the PSP's XMB, and have "Memory Stick" highlighted as one of your options. Hit Triangle and then "Information." If MagicGate, Sony's proprietary copy-protection scheme isn't supported, chances are high that you have a fake.