There isn't an 'automatic' method, because we don't know what rotations have been applied when importing a model.
But there are a bunch of different methods for aligning a model in some way. Here is one:
Identify a face on the model that you want aligned facing one of the global axes. Select that face and align your view to the face in the orientation you want it using the Shift+Numpad short cuts or the view menu. Aligning your view to the Top of the face (Shift+Numpad 7) will result in that face being aligned to global Z when you're finished (use Left and Front views if you want to align to X or Y).
Snap your cursor to selection and keeping your view aligned switch back to object mode and add an object (a cube is fine). Make sure to choose 'Align: View' before messing with anything else.
Aligning the new object to View will give it the transformation rotations that the face would have to have gone through to be where it is compared with the global 'world' orientations. You can now simply select your original object, then the new object and join them (Ctrl+J). Then clear rotations and it will be aligned to what ever global axis you wanted. Obviously you then delete the cube in edit mode.
There is some way to do this using custom transform orientations (2.8: The + button in the transform orientations drop down) instead of adding an object. But I don't remember how, so maybe someone else can post that method.