How do I check that a Thai marriage certificate is genuine?

The safest way is through the Consulate General of Malaysia in Songkhla and your state religious affairs office. A genuine certificate is issued by a legitimate Provincial Islamic Council in Thailand. Endorsement by a Malaysian mission is the usual step before registration in Malaysia. Religious affairs officers can tell you if certificates from a particular source have had problems before. Warning signs include an agent who refuses to give the officiant's details, no record at the religious council, or a contract without a valid wali. If in doubt, do not proceed and get advice from the religious affairs office first. See Thailand.