Take the URL normally used for anyone's profile (for example, mine is http://www.whatifsports.com/user/profile/?user=caesari) and replace the username at the end with gibberish, or any word that you don't think would be used as a username, and see what happens!
Some letter combos to try (typed completely randomly):
fakdjqqqffffarfysucksfffjkln
jkllfffarfyisadouchefffnkij
jaskerfffArfyisafemalefffiarkl
^^None of these are usernames, which fits the criteria! Now give it a try!