As of Monday, annual checkups for the nearly half a million Massachusetts children on Medicaid will carry a new requirement: Doctors must offer simple questionnaires to detect warning signs of possible mental health problems, from autism in toddlers to depression in teens. The checklists vary by age but ask questions about children's behavior - whether they are spending more time alone, seeming to have less fun, having trouble sleeping - that are designed to trigger discussion between parents and doctors. The conversations may or may not lead to a referral to a specialist