I agree with iwire, test by either using a known AC voltage source, or by rubbing on your sleeve or other part of your clothing, before AND after checking the terminal in question. If there is any chance it may be falsely registering no voltage, for example due to a preponderance of grounded metalwork, I would revert to a contact voltmeter of some kind. But generally, I have found the non contact kind very reliable if treated well. I was always taught that the 'prove your tester actually works both before and after testing an unknown conductor/terminal' routine, and it makes sense to do so with both non contact and traditional testers.
