The classic teaching for the Boards exam is
A case series of 8 pediatric patients were documented with coins positioned in a sagittal plane were actually in the esophagus! This data was collected over 15 years.
The classic teaching likely still holds true most of the time (sagittal coin = tracheal foreign body), but don't rush to immediate judgment. Take a look at the lateral view, and see where the coin is located with respect to the trachea and airway. It may be more posterior, in which case, it's in the esophagus.
Thanks to Dr. Matt Anderson (Resident at Univ. of Wisconsin EM program) for the tip!
Schlesinger AE, Crowe JE. Sagittal orientation of ingested coins in the esophagus in children. AJR Am J Roentgenol. 2011 Mar;196(3):670-2. Pubmed .