Asynchronous motors have a simple and reliable design, but they have a limitation on the efficiency class. Synchronous motors with direct start from the mains with or without permanent magnets are an energy-efficient alternative to traditional asynchronous motors in a wide range of applications with direct power from the mains. However, the developers of such engines face a lot of difficulties, one of which is the lack of reliable and accurate testing methods for such electric machines. In this paper, we propose a method for indirect determination of efficiency for synchronous motors with direct start from a network with magnets by summing up the individual components of losses determined during standard engine tests: idle and load experience. Unlike the standard direct method for evaluating the efficiency of an electric motor (input-output method), which is usually used for these machines, the proposed method does not require direct measurement of torque and mechanical power on the shaft, which require expensive and prone to failure measuring equipment, such as a torque sensor and a speed sensor. The theoretical justification of the proposed method and its experimental verification using a mass-produced 4-pole synchronous motor with direct start from the mains with a rated power of 0.55 kW are given. The satisfactory convergence of the experimental results obtained using the standard input-output method and using the proposed indirect method is shown. This method can be used as the main one in case of impossibility of direct measurement of the engine torque or as an auxiliary one if it is necessary to check the results of measurements by more than one method.