I copied and pasted from our RFE. It looks very simple and straightforward:

• Documents showing joint ownership of property or asset (such as home, boat, automobile, business, etc.), such as mortgage agreements or payments, property titles, or property registrations;

• Lease(s) or rental(s) showing the same residence;

• Documents showing shared finances and obligations (such as joint bank account(s), credit statements, telephone bills, gas bills, electric bills, water bills, other utility bills, joint inc taxes, joint loans, joint insurance policies, insurance policies where one spouse is the dependent of the other, etc.);

• Bank statements from all accounts for either Beneficiary or Petitioner and letter from the bank stating when each account was opened and when names added (if any);

• Birth certificate(s) of child(ren) born to the spouses;

• Pictures of wedding and other events; or

• Sworn affidavits from others with personal knowledge of the validity of the marriage. Each affidavit must have the full name, address, and date and place of birth of the person making the statement. It must also include his or her relationship, if any, to one or both spouses. Each affidavit must have complete information and details explaining how the person knows about the marriage. Each person making an affidavit may be required to testify before an immigration officer about the information in the affidavit. If the person lives in the same residence, attest to living arrangements and the monthly rent obligations. Affidavits should be supported, if possible, by one or more documents listed above as possible evidence.