How can you tell the difference between bad constipation and a bowel obstruction?

One need immediate. medical attention , bowel obstruction is pathological ( disease ) process needs medical intervention including possible surgery , dissension due to constipation is due to inability to eliminate stool , still will be able to pass flatus .
See below. Presentation can vary. The pain from a small bowel obstruction tends to be severe, accompanied by vomiting, &high-pitched tinkling bowel sounds. With constipation,depending on cause, bowel sounds may be absent.This is NOT a diagnosis best made at home, however; severe constipation can become medically serious, &if it has reached the point of severe pain, vomiting, etc, ER evaluation is necessary.