The US Department of Housing and Urban Development (HUD) has published guidelines they say will make it easier to understand what type of proof property owners can require from renters showing they own a legitimate service animal.  In some cases service animals are exempt from no-pets policies.

The directive spells out the difference between assistance animals, service animals and support animals; what questions landlords can ask; and under what circumstances they can require documentation.  Although 19-pages long, it is fairly user-friendly.