Wis. Stat. § 889.24: Conveyance, how proved.

When any grantor shall die or depart from or reside out of this state, not having acknowledged the grantor’s conveyance, the due execution thereof may be proved by any competent subscribing witness thereto before any court of record; if all the subscribing witnesses to such deed shall be dead or out of this state the same may be proved before any such court by proving the handwriting of the grantor and of any subscribing witness thereto.