To do this you can make a "small maneuver" using the method getDirty
.
This method serves to bring the attributes that were changed before the model update.
So you could do like this:
$usuario = Usuario::find($id);
$usuario->fill(Input::all());
$campos_alterados = $usuario->getDirty();
$usuario->save();
I think this example is the simplest way. But we can also consider using some PHP OOP features.
For example, we can clone the object before changing it and compare the differences:
$usuario = Usuario::find($id);
$clone = clone $usuario;
$usuario->update($inputs);
array_diff($clone->getAttributes(), $usuario->getAttributes());