Nothing!
You have to rely on that the exchange student agency weight safety over income.
However a number of cases clearly inform us about not only isolated incidents but a pattern of neglect.
Child advocacy groups has published an article naming a number of cases where young exchange students have been raped or otherwise sexual molested by the people they should have their trust in while they are abroad. We are talking members of the host family, school teachers or counselors.
How are you going to ensure your child's safety when it is 1,000 of miles away from your home?
Ask questions - also the difficult ones before it is too late.
References:
Exchange student organizations headed for court (Child advocacy, March 1 - 2006)