When a party to a suit, his agent or attorney, shall make oath that the residence of any party defendant is unknown to affiant, and to such party when the affidavit is made by his agent or attorney, or that such defendant is a transient person, and that after due diligence such party and the affiant have been unable to locate the whereabouts of such defendant, or that such defendant is absent from or is a nonresident of the State, and that the party applying for the citation has attempted to obtain personal service of nonresident notice as provided for in Rule 108, but has been unable to do so, the clerk shall issue citation for such defendant for service by publication. In such cases it shall be the duty of the court trying the case to inquire into the sufficiency of the diligence exercised in attempting to ascertain the residence or whereabouts of the defendant or to obtain service of nonresident notice, as the case may be, before granting any judgment on such service.
Tex. R. Civ. P. 109