Standard method to differentiate between endotracheal and esophageal intubation is-

Auscultation is the most commonly used method to detect endobronchial intubation. But it may be misleading, as breath sounds can be transmitted to opposite side of the chest. Chest x-ray is reliable but time consuming and expensive. The radio opaque marker at the patient end shoilube in the middle third of trachea. End tidal CO2 is the most reliable and standard method to detect endobronchial intubation.