Fastest way to check for bad sectors in HDD?

You have to check some SMART utilities.

You can use "Disk Utility" (aka palimpsest or gnome-disks depending on the Ubuntu release) for this.

In command line I suggest you install the smartmontools package (sudo apt install smartmontools) and play around with smartctl.

Example:

sudo smartctl --all /dev/sda

If your drives have SMART, I would go about this by installing "gsmartcontrol" and doing a short test on all drives. Then throw away the ones that do not even pass the short test or display errors/warnings and perform a long test on the remaining drives.

To truly find out if a drive is damaged or not, I guess you will need to check every sector of it and that's what the long test does.

But I am no expert, maybe there is a better way?