Illustrator CS6 has a bounding box for every object in the artboard, if you rotate an object then its respective bounding box is affected in such a way that (provided you have not reset the bounding box after said rotation operation) you can get the object back to an "un-rotated" state.
I've just confirmed this with illustrator 9. The rotated bounding box provides a "cartesian memory" for rotations.
- Pull guides to the 2 corners that define the lowest edge of the bounding box.
- Use the bottom edge of the bounding box as a reference.
- The angle of rotation is the angle between the x-axis and the line made by the bottom edge of the bounding box.
*Please note that the origin of the X-axis described above is the
lowest corner of the bounding box.