You would need a multimeter. Set it to check voltage. Put the batteries down inside the extension tube but don't attach it to the PV. With the batteries in the tube, touch the red lead to the positive terminal of the top battery, and the black lead to the body. If they are fully charged 3.2v batteries you should get at least 6.4v reading or a little over. If it shows nothing, then it isn't grounding.
Now even if you do get a perfect reading, if the spring is flattened down to far, it may not be pressing the batteries up enough to create a solid connection to the positive terminal. When you screw the switch section onto the extension, can you feel an obvious resistance from the batteries pressing against the positive terminal in the switch assembly?