A non-destructive on-site method of inspection, and testing of the insulating performance and reconditioning of a concentric insulating tubular conduit member is disclosed. The insulating tubular conduit members comprise concentric walled members with an intermediate annular insulating cavity defined therebetween. This non-destructive method of testing and inspection is based on a determination of the presence of an unacceptable level of moisture in the annular insulating cavity. Visual, auditory, electrical, and radioactive indicating means can be employed in accordance with the principles of this invention. The use of a vacuum pump to establish at least a partial vacuum in the annular insulating cavity in conjunction with the testing technique employed herein, additionally provides a means for reconditioning and improving the insulating performance of insulating tubular conduits and permits reassembly of a fluid transmission conduit in which each of the tubular conduit sections will have the desirable insulating capacity.