If you are in a state that recognizes common law marriage, you can show proof of your common law relationship by providing affidavits from two people who are aware of your common law relationship and an affidavit from yourself regarding your history together.
How long the two of you have lived together.
A listing of addresses where you lived together.
Any awareness of any public announcement of your common law marriage.
If you were considered as husband and wife among your neighbors, friends, and relatives.
Your own affidavit should also state:
The state where the two of you agreed to be husband and wife.
The date when you made this decision.
Any previous marriage relationships, both licensed and under common law. If there are any such marriages, you will need to provide wedding dates, and how the relationships ended.
The following documents may be helpful in proving your common law marriage: naturalization certificate, immigration record, insurance policy, deeds, passport, child's birth certificate, bank records showing joint accounts as husband and wife, church, employment, and other records.
Remember than all affidavits must be notarized.
Providing false information, or concealing important facts can be considered a violation of the law and may be punishable by a fine, or imprisonment, or both. (18 U.S.C. 1001)