This paper presents an algorithm to calculate mechanical stability margins of a Modular Snake Robot (MSR) during scouting poses. Scouting poses are defined as robot configurations in which one or two of the end modules of the robot are raised up to increase the range of perception of sensors that might be placed in its distal parts. The robot center of mass (CoM) and each of the module's contact pad positions are calculated by computing the robot kinematics. Then, this kinematic model is placed in an environment that consist of a height map (the terrain), built on a 2D grid base of defined size and resolution. Due the hyper-stability of the MSR structure, as it features many static contact points with the terrain, we approximate by weighting the distribution of forces to ensure an iso-static simplified problem. Using this information as input, the algorithm calculates a representation of the supporting surface (not necessarily horizontal), and then, it computes the minimum distance of the CoM projection into this surface to one of its edges to define a mechanical stability margin. The effectiveness and robustness of the method is demonstrated by comparisons of simulation and the real robot results. Moreover, a sequence of quasi-static motions bounded by a threshold in the stability margin, keeps the robot stable as it rises. Thus, demonstrating qualitatively the convenience of the method.