Increasing logic densities and clock frequencies on FPGAs lead to rapid increase in power density, which translates to higher on-chip temperature. In this paper, we investigate the thermal behavior of general applications on fine-grain reconfigurable fabrics and we introduce the pre-mapping sensor insertion problem for thermal monitoring. Our study shows that on average the maximum temperature on the chip is 19.5degC higher than the ambient temperature for a transition density of 0.5 at the primary inputs. For fine-grain reconfigurable devices targeted for general applications it is difficult to predict the locations of potential hotspots a priori. However, programmability presents a unique opportunity for effective thermal monitoring. It would allow us to perform a thermal simulation on a given design first and obtain the locations of potential points of interest in a design. Then, in the pre-mapping stage the design can be updated with insertion of thermal sensors. Given a set of expected hot spots in a design we aim to determine the minimum number of sensors and their locations in order to monitor these locations with a given sensitivity requirement. Since the thermal sensors are implemented using unused CLBs on the fabric it is essential to use the logic resources efficiently. We propose an efficient algorithm to solve the sensor placement problem addressing this optimization goal