Embedded and cyber-physical systems are pervading all aspects of our lives, including sensitive and critical ones. As a result, they are an alluring target for cyber attacks. These systems, whose implementation is often based on reconfigurable hardware, are typically deployed in places accessible to attackers. Therefore, they require protection against tampering and side-channel attacks. However, a side-channel resistant implementation of a security primitive is not sufficient, as it can be weakened by an adversary, aging, or environmental factors. To detect this, legitimate users should be able to evaluate the side-channel resistance of their systems not only when deploying them for the first time, but also during their entire service life. The most widespread and de facto standard methodology for measuring power side-channel leakage uses Welch’s t-test. In practice, collecting the data for the t-test requires physical access to the device, a device-specific test setup, and the equipment for measuring the power consumption during device operation. Consequently, only a small number of cyber-physical systems deployed in the field can be tested this way and the tests to reevaluate the device resistance to side-channel attacks cannot be easily repeated. To address these issues, we present a design and an FPGA implementation of a built-in test for self-evaluation of the resistance to first-order power side-channel attacks. Once our test is triggered, the FPGA measures its own internal power-supply voltage and computes the t-test statistic in real time. Experimental results on two different implementations of the AES-128 algorithm demonstrate that the self-evaluation test is very reliable. We believe that this work is an important step towards the development of security sensors for the next generation of safe and robust cyber-physical systems