How can I rotate a 3D model to be parallel with the build plate? I have an STL that has a flat surface which should contact the build place when 3D printing. The model has an unknown rotation. How can I rotate the 3D model to be flush with the build plate?

I am very comfortable with OpenSCAD, and I can use it to rotate the STL if I know the correct angle. Is there a tool that will let me analyze a single face (triangle) of a model to determine it's normal? If I can even get the coordinates from 3 points from the desired face I can calculate the normal and use that to calculate the rotation vector.
Some tools like Cura or Repetier Host have slicers that analyses and tell you if it is ok to print or not. Both of them allow rotations.