I always do this with new USB sticks. Write one random file, fill the drive with copies and then test for the checksum, for example for a 16 gig thumbdrive:

    $ cd /path/to/thumbdrive
    $ dd if=/dev/urandom of=test.bin bs=1000000 count=100
    $ for i in $(seq 1 160); do echo "Writing file $i"; cp test.bin test$i.bin; done
    $ md5 test.bin
    MD5 (test.bin) = 0d3c4fe338109b09b61fcea3e11b0e4b 
    $ ls *.bin | xargs md5 | grep -v 0d3c4fe338109b09b61fcea3e11b0e4b

I'd be happy to hear if there's a oneliner for this, though.