Disk utility in command line
In fact, Disk Utility does use fsck to check the disk in question. "Verify Disk" uses
/sbin/fsck_hfs -fn -x /dev/diskX
while "Repair Disk" runs
/sbin/fsck_hfs -fy -x /dev/diskX
These are the options used:
-f: 'f'orces a check, even if the disk seems to be clean
-n: 'n'ever attempts to repair any found issues
-y: says 'y'es to any question whether an issues should be repaired (as you might have guessed, -n and -y are exact opposites)
-x: generates output in XML format, which is parsed by Disk Utility
/sbin/fsck (as opposed to /sbin/fsck_hfs) is a wrapper that calls /sbin/fsck_XXX for the type of file system in question. You'll get more information in the man pages for fsck and fsck_hfs, from which I've borrowed a lot of information for my reply.
/usr/sbin/diskutil
is what you are looking for.
Checkout man diskutil
and look at the repairVolume
command.