When applying for a K1 fiancé visa, both the sponsoring spouse and the spouse seeking a green card need to prove they each have the “legal ability” to get married.
For the U.S. citizen, proving legal ability to marry is straightforward. They simply attach divorce and/or death certificates to their visa petition for each of their previous marriages. The divorce decree and/or death certificates need to be official copies with the stamp of the granting court or government office.
The fiancé needs to provide the same evidence regarding divorce or death of a spouse. This area is a little trickier since every country handles these matters differently. Adjudicators will be looking closely at these documents to determine authenticity. U.S. Citizenship and Immigration Services (USCIS) refers to this as “proof of the legal termination of the previous marriage of the beneficiary.”