Before any warrant shall issue for lands within the limits aforesaid, the person in whose name or for whose use such warrant is applied for, shall, in addition to the proof already required by law, prove, by a disinterested witness, whether the said lands are improved or unimproved, and if improved, the date when such improvement was commenced, that interest may be charged accordingly; which proof shall be taken, on oath or affirmation, before two justices of the peace¹ of the county in which the lands applied for are situated.