(a) The deceased spouse's signature to an agreement that is the subject of an application under Section 112.101 may be proved by:
(1) the sworn testimony of one witness taken in open court;
(2) the affidavit of one witness; or
(3) the written or oral deposition of one witness taken in accordance with Section 51.203 or the Texas Rules of Civil Procedure.
(b) If the surviving spouse is competent to make an oath, the surviving spouse's signature to the agreement may be proved by:
(1) the sworn testimony of the surviving spouse taken in open court;
(2) the surviving spouse's affidavit; or
(3) the written or oral deposition of the surviving spouse taken in accordance with Section 51.203 or the Texas Rules of Civil Procedure.
(c) If the surviving spouse is not competent to make an oath, the surviving spouse's signature to the agreement may be proved in the manner provided by Subsection (a) for proof of the deceased spouse's signature.