A fluorescent tube with gas will light dimly when touched by the Tester's aerial, but a tube without gas will remain dark. However, even if the tube lights up, there still may be continuity problem. The Tester's pin socket reads the continuity at each end of the lamp. If the indicator light glows, all's well. If not then the cathodes or pins are faulty.
The Lamp Tester aerial can also be used to check a lamp's color temperature or a visual effects color, such as when many tubes are mixed together in a case.