You could check in 6 directions (like jfk EO-11110 said), and when the test is positive (the linepick hits the outer object), you could check with MeshesIntersect.
If that returns False, then your inner mesh is really inside the outer mesh (this could work for the example Floyd mentioned, cylinder inside torus).
If that returns False, then your inner mesh is really inside the outer mesh (this could work for the example Floyd mentioned, cylinder inside torus).