Acute abdominal series
To differentiate Large and small bowel, remember:
1) Large bowel have haustra which appear as lines 1/3-1/2 of the way through the bowel, but not all the way through.
2) Small bowel have lines (plica circulares) all the way through the bowel
Acute Abd XR have poor sensitivity and specificity for SBO...if you are really concerned, get the CT.