OSHA requires in 1910.135(b)(1)that protective helmets (hard hats) purchased after July 5, 1994, must comply with ANSI Z89.1-1986. The 1997 edition of the ANSI standard says that hard hats and all components should be visually inspected daily.
Look for such things as signs of dents, cracks, penetration, and any damage due to impact, rough treatment, or wear that might reduce the degree of protection originally provided.