If there is free intra-abdominal gas adjacent to a gas-filled loop of bowel then both sides of the bowel wall are well-defined. This is known as 'Rigler's sign'.
This image shows a tiny triangle of gas outside the bowel, indicating pneumoperitoneum. Even this tiny volume of free gas is potentially life-threatening in the context of suspected perforation.
If in doubt make sure there is a good quality ERECT chest X-ray available.
If perforation is suspected an erect chest X-ray is more sensitive than an abdominal X-ray for detection of free gas.
Occasionally Rigler's sign is visible on the erect chest X-ray. This image shows free gas under the diaphragm which outlines the outer side of the bowel wall.