What about driving licences?
A driving licence with a motorcycle endorsement is necessary for bikes of 250cc or more. An International Driving Permit is also preferable. For automatic scooters, we do not insist on a licence but require your assurance that your are competent to ride.
Update 6/2016: Police are increasingly asking for IDP if stopped. This is not a legal requirement as you are a tourist on holiday. However, they have you by the short and curlies so best to smile and if asked, pay the spot fine which is normally 500 baht.
More info here: http://driving-in-thailand.com/type-of-driving-licence-required/