Why Mac Disk Utility Run so fast?
The OS X disk utility checks the HFS+ volume header, and its backup header (located in the last block of the disk). If both match, then the rest of the disk is assumed to be consistent. That's along the lines of quick fsck(8) check. Another quick check for journal consistency is performed if you've set up journaling, and if that also checks ok, then the rest is skipped.

You can always run fsck manually, or - http://www.newosxbook.com/ has a tool called HFSleuth which - while directed at programmer/hacker types - can allow you to do a more thorough check.