Shafts are often subjected to difficult operating conditions in high-performance rotating equipment such as compressors, steam and gas turbines, generators and pumps. As a result, shafts are susceptible to fatigue failures due to transverse cracks. In this study, vibration monitoring and orbital paths observation were used to detect the presence of a flaw in a shaft. Two types of flaws were tested: a straight slot, and a fatigue crack. For both flaw types, specimens of different depths were examined in order to assess the detection capability. A new approach to examine vibrations at the critical speed is proposed; this speed is chosen because of the strong connection to the basics of the physical problem. Orbital paths are suggested as means for fault detection as well. The presence of a straight slot in the shaft was found to be related to a decrease in the natural frequency and to a decrease in amplitude of the first order at critical speed. For the fatigue crack, a consistent trend in critical speed and in amplitude was not seen as crack depth grew. A new method to detect the change in the shaft natural frequency is proposed. The combination of two indicators, change in critical speed and change in amplitude at critical speed, are suggested for classification of flaw size. For the straight slot case, the method proposed was able to distinguish between different fault depths.