We report a non-direct method to determine the minimum conductivity (sigma(min)) of graphene. The proposed technique is based on the prior characterization of graphene sheet impedance at microwave frequencies using waveguides, and on the subsequent extraction of sigma(min) from the measured scattering rate. This approach presents some advantages as compared with standard DC characterization methods, as it is inherently contactless and does not require graphene patterning. Our experimental study estimates a sigma(min) in the range of 2e(2)/h-4e(2)/h for different samples. We also compute the density of charged impurities of our samples, and show that they are related to graphene conductivity at its Dirac point.