The best would be to have a revamped combat text (like the wow add-ons plz T_T) with 2 different colors for physical / magical damage taken.

You can look at the combat text altough I highly recommande you make a special tab with only the damage YOU take and nothing else. Then you can easily check.
Finally as many people have said, you can look at the animation but it's it always clear and there are some elemental effect that can't be weaken (holy and dark for instance) by skills like reprisal (I think that's the skill name I'm not sure)