Most non-intrusive induction motor (IM) efficiency estimation methods rely on the machine slip to compute the output power and the efficiency. To achieve the overall non-intrusive nature of the entire process, it is vital to utilize a reliable and non-intrusive rotor speed measurement technique in order to obtain the slip of an in-service motor. A number of rotor speed estimation techniques have been proposed in literature. This article focusses on the review and analysis of different techniques in terms of intrusiveness, relative accuracy, sensitivity and computational overhead particularly when the IM is fed by an inverter supply. An experimental accuracy and sensitivity analysis of some motor current and vibration-based techniques is carried out to check the accuracy trends reported in literature and the robustness of these techniques. Potential techniques for non-intrusive rotor speed estimation of inverter-fed induction motors are recommended.