My husband says his second marriage has been registered. How do I know that is true?

The simplest way is to ask your husband to show you the marriage certificate for that marriage, because registration protects everyone's rights, including yours. A registered marriage has a certificate issued by the state Islamic religious affairs office. If he refuses, ask him calmly why. The religious affairs office may not disclose other people's records freely, but you can ask about the proper way to verify it. If a dispute arises later about maintenance, the rota or inheritance, a syarie lawyer can advise how to obtain the information through the court. This clarity makes your family planning easier. See if polygyny was without permission.