Basically, if you had a bowel obstruction, you would not be passing any stool. You would have severe stomach pains, you would not be able to open your bowels and due to this your stomach would probably be quite swollen. You may also be vomiting fecal fluid. If you have any of these symptoms then you need to go to the ER straight away.
Other than that, pale stools can be caused by what you eat, or can indicate possible gallbladder problems.