Detecting if a mesh is inside another?

You could try flipping the second mesh, the one you think you are inside, and then testing the assumption with a linepick from inside the first mesh to somewhere you are sure will be outside the second. That's assuming the potential containing mesh is completely closed.

Edit... thinking about it that will not work too well. You could perhaps create a duplicate of the second mesh and flip it, so you can be sure you are picking inside out and not outside in.