for testing external HDD with smartctl,

install smartmontools

$ sudo apt-get install smartmontools

$ sudo smartctl -t short -d sat /dev/sda
$ sudo smartctl -t long -d sat /dev/sda -T permissive

then it will say

Testing has begun.
Please wait 100 minutes for test to complete.
Test will complete after Thu Oct  6 11:36:22 2016

After its completion,

$ sudo smartctl -l selftest -d sat /dev/sda -T permissive

to see the test result.