How do you show proof of marital status without or marriage certificate or name change?

Merely having the same last name as your spouse is only informal proof that you are married. It is not legal proof that you are married. Many people do not change their last names anymore. The only legal proof of marriage is a marriage certificate. In certain very rare instances, you may be able to substitute sworn affidavits of persons who attended your wedding. One example would be a refugee in extraordinary circumstances. Otherwise, you will be expected to obtain a copy of your marriage certificate.