We report results of measurements on SensLAB, an indoor wireless sensor network testbed with a large number of nodes equipped with a CC1101 radio chip. We analyze RSSI and LQI to find the best way to discriminate good links from weak ones and in particular, to derive a metric that estimates the Packet Reception Ratio (PRR) the best. To obtain an estimator of PRR, we have fitted a Fermi-Dirac function to the scatter diagram of the average and standard variation of LQI. The function enables us to find PRR for a given level of LQI. We evaluate the estimator by computing PRR over a varying size window of transmissions and comparing with the estimator.