Steel-fiber-reinforced concrete is a composite material which is becoming more and more widely employed in building construction, due to its better resistance to cracking and to crack propagation with respect to plain concrete. Its mechanical behavior strongly depends on the choice of fiber properties and their volume fraction in the concrete mixture. As any material used in building construction, the “on-site” test of its properties represents a very important task. Often, this requires the employment of noninvasive and nondestructive measurement procedures that have to be carried out directly in situ. In this paper, a new method for the estimation of the fiber density and their average orientation is presented. The measurement technique is based on the employment of a probe that is sensitive to the magnetic properties of the steel fibers. The performance of the method has been theoretically and experimentally analyzed.