How to use fsck in Ubuntu?

To run fsck on your harddrive you need to boot a liveCD and then run the commands with your harddrive partitions as the target.

"There is no /forcefsck file in my root"

-You need to create the file, it is just a blank file so run sudo touch /forcefsck and fsck will check your drive next time your reboot.

I concur with @bodhi.zazen comment here. The best way to go about checking a filesystem is to boot from live media, choose "Try Ubuntu" and then manually fsck the partition in question. For example sudo fsck /dev/ZdXY where ZdXY is the partition in question. sudo fdisk -l will give you a listing of your drives and the partitions on them. This information should be sufficient for you to determine which partition you need to check.