Buy from a reputable source and you should be fine.
For the paranoid there
free testers available that will determine if your a card isn't performing up to spec'.
....
I was looking for this. I buy generally on the internet, some have been e-bay, some amazon. Cheap is first priority, then I look at speed.
My latest Kingston U3 90 MByte/s write 80 MByte/s read:
Warning: Only 30611 of 30612 MByte tested.
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 20.0 MByte/s
Reading speed: 1.07 MByte/s => I had a better result on an other computer
H2testw v1.4
=> very disappointed
----------------------------------------------------------------
Lexar 1000X UHSII
Warning: Only 32000 of 60063 MByte tested.
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 22.6 MByte/s
Reading speed: 32.6 MByte/s
H2testw v1.4
----------------------------------------------------------------
Lexar 600X UHSI micro
Warning: Only 10000 of 30531 MByte tested.
Test finished without errors.
You can now delete the test files *.h2w or verify them again.
Writing speed: 19.8 MByte/s
Reading speed: 32.5 MByte/s
H2testw v1.4
=> convinced I`m doing something wrong in the testing. Speed is generally not blocking in photography at the moment, but I try to get what I pay for.