Don't need to open it up physically though.
Using a set of black-box circuit analysis measurements you can find out a great deal about it, enough to determine its usefulness in the intended circuit configuration.
If that´s really how it goes about the windings, you will want to unsolder the grounding connection of the upper-left pin to ground, so that it works as it should with isolation and proper loading.
rafafredd, what picture are you refering to? The correct one is the drawing, the other pics are incorrect... centertap is the pin in the middle of the 3 pinned row