For the use in rehabilitation robotic devices, various types of actuators are currently being investigated. Variable impedance actuators seem promising concerning individualized training, comfort and safety. For the control of actuators, some sensors are required, however, to keep the rehabilitation device light, also the implementation of measuring instruments should be reduced. Thus, in this contribution, we introduce three different ways to observe the interaction torque of an electrical actuator. Using a detailed model of an actuator and mechanical system, the interaction torque is investigated through different sensor signals. In a real actuator, non-linearities appear. Thus, for an accurate estimation, an unscented Kalman filter is promising. For serial elastic actuators, the deflection can be directly quantified on the compliant element using strain gauges. Another way is to detect the deformation of compliant elements using two angular encoders. All ways to sense the interaction torque are analyzed for a sinusoidal excitation.