The Senate has been called upon to judge the return of an appointed Senator at least twice.
In 1893 (before the 17th Amendment provided for the direct election of senators), the Montana legislature adjourned without electing a senator. After the adjournment, Gov. John E. Richards appointed Lee Mantle to fill the vacant seat. By a three-vote margin, the Senate determined that a vacancy the legislature knew about and did not fill was not the sort of vacancy that the governor had the power to fill himself. Accordingly, Mantle was denied the seat.
In 1913, just after the 17th Amendment was ratified, Sen. Joseph Johnston of Alabama died. The Alabama legislature was in recess, and the governor appointed Frank Glass to fill the vacancy. The last provision of the 17th Amendment reads, "This amendment shall not be so construed as to affect the election or term of any Senator chosen before it becomes valid as part of the Constitution." If this clause was read as attached to the Senate seat, then the 17th Amendment did not yet apply to it, and Glass was properly chosen under the pre-17th Amendment procedure. If, on the other hand, the amendment applied to the individual senator, then it was operative with regard to the Alabama seat. In that case, the appointment of Glass was unconstitutional, and the seat would have to be filled by special election. The Senate determined that the latter was the correct interpretation. Glass did not get the seat, which was filled instead by a special election.