Using hdparm to measure a hard disks performance.
To measure the current performance of your disk run:
hdparm -tT /dev/sda
You will get something like:
/dev/sda:
Timing cached reads: 896 MB in 2.01 seconds = 446.06 MB/sec
Timing buffered disk reads: 168 MB in 3.01 seconds = 55.77 MB/sec