Modern high-performance processors employ thermal management systems, which rely on accurate readings of on-die thermal sensors. Systematic tools for analysis and determination of best allocation and placement of thermal sensors is therefore a highly relevant problem. This paper proposes a novel technique for determining the placement of temperature sensors on complex Multi-Processor Systems-on-Chips (MPSoCs) floorplans. The proposed method first analyzes the observability of the system for all the possible sensor placement configurations. Minimum sensors placements ensuring the observability of the portion of the MPSoC system that is relevant to the designer are then compared with simulation-based data coming from a wide set of benchmarks. Pareto points identifying the best configurations are than stored. According to user designer needs the best configuration is selected and a specific location is assigned to each sensor. We compared the proposed method with state-of-the-art approaches [5], [6]. Results show a reduction up to 4.5x in the number of required sensors.