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.