To ensure a helmet is in good condition you should:

• A. Check to make sure it has not been dropped.
• B. Only purchase a new one. Never buy a second hand helmet.
  Explanation:

  "You don't know whether a second hand helmet has been in an accident or has been exposed to solvents or other chemicals that might have weakened it. You can't tell by looking at a helmet whether it has microscopic cracks that might weaken it."
• C. Only buy a helmet that someone else has tried out first.