Bumped another car at 20mph, everything seems ok - what should i check?

My husband's 325i bumped into another car from behind while they were on the expressway in slow traffic. He said he was going about 20, and the other car was moving, but slower. He bumped their left rear bumper with the center of his front bumper. There are some minor signs of stress on the outside, but aside from being very shaken from the experience, everything seems to work.

Is there something we should check aside from trying all the widgets and driving around a little? Does this have to go to a mechanic?