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Calcaneus Fractures:
Calcaneus fractures can easily be missed on plain films and the true extent of the injury might not be appreciated until a CT is done. However, you can increase your change of picking up a calcaneal fracture by evaluating Bohler's Angle.
Lateral radiographs of the foot are needed to evaluate the Bohler angle. This is the angle made by drawing a line from anterior process of the calcaneus to the peak of the posterior articular surface and a second one drawn from the peak of the posterior articular surface to the peak of the posterior tuberosity. (See Picture) The average angle is 25-40°. Angles less than 25' are strongly suggestive of a fracture and the patient should probably get a CT of their foot if there is clinical suspicion.
References
Photo and additional discussion can be found at http://www.blackburnfeet.org.uk/hyperbook/trauma/calcanealFx/imaging.htm and on eMedicine -- Calcaneus Fractures by Scott Nicklebur, MD http://emedicine.medscape.com/article/1232246-overview