If they are in front of you watch the front wheels and the driver for clues. My hearing is not so great (lots of shift work in factories while in college) so I already do this for normal cars. For example usually a car about to leave a paralel spot will turn there wheels to the left before they start to move. For a car behind you, other than a mirror there isn't much you can do. I have a coworker who seems to take great joy in sneaking up behind me in his Prius and laying on the horn, he always comments on how high I jump out of the saddle; I'll admit its gotten old at this point, but I can imagine it is amusing to watch me jump up like a startled white tail deer.
Overall I like Prius drivers they seem to drive with less anger than the rest of the bozos on the road. YMV