In the usual situation you should closely inspect the film for distended loops of bowel. Sometimes there is little gas and a lot of fluid so the distension is not obvious. If you see distended loops of bowel, look and see if you can see gas in the rectum. If there is gas in the rectum then the patient may have a pseudo-obstruction or an incomplete obstruction. If the rectum appears empty, look for gas in the sigmoid and so forth, try and identify where there is a transition zone from collapsed to distended bowel, that is the site of the obstruction.
If there is a single huge gas filled loop like a rubber tyre then there may be a volvulus, the most likely location is a sigmoid volvulus. The loop points away from the mesenteric attachment at the base of the twist so in the case of a sigmoid volvulus the base of the twist is in the left iliac fossa and the loop points towards the right shoulder. In a caecal volvulus the base of the twist is in the right iliac fossa and the loop points upwards or to the left shoulder.
If there are many distended loops of bowel, it is most likely distended small intestine, look for the stack of coins appearance of the valvulae conniventes. The ileum is somewhat less featureful than the jejeunum. Distended small bowel often lies in the center of the abdomen.
Distended colon can be identified by observing the folds of the haustral markings which do not fully cross the diameter of the bowel.