Instructions
Look at the front of the Memory Stick. If it does not display the official Memory Stick logo, it is a fake memory stick. The logo is an upper-case letter "M," with three horizontal lines underneath it, and you'll normally find it in the top-right corner of the Memory Stick.
Insert your Memory Stick into your PSP console. Navigate across to any menu that displays your Memory Stick, such as "Game" or "Music." Highlight the Memory Stick and press the triangle button. Select "Information" and look at the line that says "MagicGate." If it says "Supported," your Memory Stick is genuine. If it says "Unknown," your Memory Stick is fake.
Plug your PSP into your computer and try transferring files over to it. Nearly all fake Memory Sticks have much lower capacities than the stated one, so if you cannot transfer 8 GB of music, photos, games or videos to it before it gets full, it is most likely fake.