You can do it like that:
Code:
% dd < /dev/ad0 > /dev/null
it will read every block of your hard drive, if you get any read errors, then you have bad sectors.
You can also try to "fix" these bad sectors with
gshred from
coreutils package:
Code:
# gshred --verbose /dev/ad0
But keep in mind that
gshred will erase all your data from this drive.