1. A system for treating an eye of a patient, the eye including a fluid posterior to a cornea, the system comprising: a femtosecond laser configured for directing femtosecond energy along a path; an optical system disposed along the path from the laser, the optical system including a focusing lens and a scanner so as to scan a non-plasma-generating focus of the femtosecond energy along a path within the patient's eye, the path extending posterior to the patient's cornea within the fluid so that the path includes a first location disposed within a tissue of the eye and a second location disposed within the cornea; a sensor oriented along the path so as to sense a first signal associated with a first focus location within the eye, and a second signal associated with a second focus location within the eye; and a computing device communicatively coupled with the sensor, wherein the computing device determines a location of an interface between the fluid and a tissue of the eye in response to the first signal and the second signal.