Watching the opponents leading elbow will enable you to interpret the angle of attack. The elbow travels at least four times slower than the fist in a circular attack, and at least two times slower that the fist in a linear attack. The elbow is also far enough away to give you time to react. No matter what movement your opponent makes his/her elbow will move.
In this drill, Sifu Guiao watches Ryan's leading elbow. The first set is done at real speed, next they slow it down and finally Sifu Guiao demonstrates a real life scenario.