Best investigation for air in peritoneal cavity is:
A radiograph of the abdomen may confirm the presence of dilated gas-filled loops of bowel (consistent with a paralytic ileus) or show free gas, although the latter is best shown on an erect chest radiograph. If the patient is too ill for an 'erect' film to demonstrate free air under the diaphragm, a lateral decubitus film is just as useful, showing gas beneath the abdominal wall.
USG
Laparotomy
Laparoscopy
X-ray
Peritoneum