Identifying drives

[HelpPleas]

I want to use the dd command from a Knoppix 5.1.1 CD to clone one SATA HDD to another, larger SATA HDD.
The drives are detected as sda and sdb.
How can I be sure which is which? I don't want to clone in the wrong direction and lose all data.
The HDD I want to clone is damaged and no partitions can be found. The target HDD contains no partitions.

[HelpPleas]

OK, I figured it out.
I have to put: sudo fdisk -l
Then I get to see details of all drives, including the size in GBs. I still think I'd be stuck if I were cloning to a drive which was exactly the same size as my source drive.
Is there a definite rule, eg: the drive attached to SATA 1 is always sda, etc?