Use the following command:
hdparm -tT /dev/sda1
A good test result is this one:
# hdparm -tT /dev/sda1
/dev/sda1:
Timing cached reads: 21416 MB in 1.99 seconds = 10785.03 MB/sec
Timing buffered disk reads: 106 MB in 3.01 seconds = 35.16 MB/sec