The simple way: Look for the USA Baseball sticker above the grip.
The technical specs: It shall not be more than 33 inches in length; nor more than 2⅝ inches in diameter, and if wood, not less than fifteen-sixteenths (15/16) inches in diameter (7/8 inch for bats less than 30″) at its smallest part. Wood bats taped or fitted with a sleeve may not exceed sixteen (16) inches from the small end.