Click on the 'Members' tab and click 'Find member'. Type in that username and if that member pops up then you have the answer
another shortcut was is in any thread type @ then the members name. eg: @fatal and you should see a small box in your text box that shows you a list of users start to form with each letter you type. If that person isn't showing up in that list, they don't have an account or the username is wrong. Of course as fatal said, and also using a keyword search in the search box helps.
