vmkfstools -Q /path/to/VM_name-flat.vmdk
You must know if the disk was or Thick Provisioned . If you don't know, assume Thick for the safest results (or check your VM settings history).
Run ls -l your-flat.vmdk . Note the byte size. Divide that number by 512 . That is your number of sectors. Example: A 10GB flat file is 10,737,418,240 bytes. 10,737,418,240 / 512 = 20,971,520 sectors .