[KimOH]

If they realize you're wearing it, they will probably make it be scanned separately -- for instance, if it contains any metal that sets off the metal detectors, or if you're "randomly pulled" for additional screening and they find it. (This happened to me.) However, if it doesn't contain any metal and you're not pulled for the more invasive check, there's a good chance you will be able to wear it through the metal detector.

If you choose to take it off for scanning, you can put it inside your carry-on bag and send it through the scanner inside the bag. However, if airport security finds it, they will insist that it's scanned by itself. (They do this with anything unusual they find in your bag or on your person in a search, not just money belts. Last week my library book had to go through the scanner by itself.)