Run this in a terminal window (without the leading
$ python -c 'import random, time; time.sleep(2 + 8 * random.random()); \ print 1; t = time.time(); raw_input(); print time.time() - t'
Press Enter as soon as the number 1 appears. Your reaction time (in seconds) will be printed. For best results, don't do it over an SSH connection. If you can go below .2 second, then you are most probably not human.