Testing the RF functions of systems-on-chip incurs a very high cost. Built-in test is a promising alternative to facilitate testing and reduce cost. However, designing built-in test circuits that tap into the sensitive RF signal paths, in order to extract useful information for the purpose of testing, often finds the designers reluctant since it results in some performance degradation that needs to be accounted for during the design. In this paper, we study a transparent built-in test approach based on non-intrusive sensors that are not electrically connected to the RF circuit under test. The non-intrusive sensors simply monitor process variations and by virtue of this they are capable of tracking variations in the performances of the RF circuit as well. The alternate test paradigm is employed to map the outputs of the sensors to the performances, in order to replace the standard tests for measuring the performances directly. We discuss in this paper the principle of operation of these sensors and we demonstrate the non-intrusive test approach on a 65nm RF low noise amplifier.