Many jurisdictions have adopted ordinances that require locking caps on FDCs which greatly reduces the probability of tampering. Otherwise, ask the property owners to frequently check these connections.
Things such as rocks, bottles, balls and anything else that will fit in them have been found. These objects will limit water flow and reduce the effectiveness if not make it non-functional for fire protection.
Stay safe and make sure to know where these are during your inspections and pre-planning.