Filing a joint return in Montana might be evidence, but not proof of a common-law marriage.

The Montana Supreme Court has set out the elements for creating a valid common law marriage:

• First, the parties must be competent to enter into a marriage.
• Second, the parties entered into the marital arrangement by mutual consent and agreement.
• And finally third, the parties confirmed their marriage by cohabitation and public repute. In the Matter of the Estate of Ober. 62 P.3d 1114, 314 Mont. 20, 2003 MT 7 (2003).