When testimony is excluded by ruling of the examiner, a party shall be permitted to offer such testimony by dictating it into the record or by submitting the substance of the proposed testimony, in writing, prior to the conclusion of the hearing. Such an offer of proof shall be sufficient to preserve such offer for review by the commissioner. The examiner may ask such questions of a witness as are necessary to determine that the witness would, if allowed, testify as represented in the offer of proof.