The FEA program should do this for you and may represent it as bending moment per unit length. In that case you would need to integrate it over the width that you want to find the bending moment for.
If you are using Patran, you can use the Freebody tool to determine the moments at each node or along a cut (via the interface selection). You must select the nodes and associated elements to the interface option. You must also request grid point forces from the solver.
Your package should have the option to plot BM contours as well as stress contours. If its steel plate it's probably more convenient to calculate the Von Mises Equivelant Stress and compare it to the yield stress (provided that the element is not susceptible to buckling. Wood-armer moments are also handy if its a slab, as it will calculate an equivelant moment which takes into account torsion.
Alternatively you could plot the appropriate stress with section depth to calc maximum Mx,My,Mz etc. The bending moment may then be calculated by integrating the compressive and tensile stresses and establishing the lever arm with them.
