We present a physical attestation and authentication approach to detecting cheating in resource constrained smart micro-grids. A multi-user smart microgrid (SMG) architecture supported by a low cost and unreliable communications network, forms our application scenario. In this scenario, a malicious adversary can cheat by manipulating the measured power consumption/generation data. In doing so, the reward is access to more than the per user allocated power quota. Cheating discourages user participation and results in grid destabilisation and a breakdown of the grid in the worst case. Detecting cheating attacks is thus essential for secure and resilient SMG but also a challenging problem.We develop a cheating detection scheme that integrates the idea of physical attestation to assess whether the SMG system is under attack. Subsequently, we support our scheme with an authentication mechanism based on control signals to uniquely identify node subversion. A theoretical analysis demonstrates the efficiency and correctness of our proposed scheme for constrained SMGs.