120v AC to 220v AC converter, most cost effective way
To answer your actual question "How would I determine which jumper to remove on my own?", simply measure resistance starting from any pin on the power plug and working your way to the other. If you see definite midpoint then that would be the one to move one power wire to.

Note, that the disconnected winding will now act as secondary and have full 120V on its terminals so, be careful with that.

Another thing to consider is heating of the transformer - with this change the primary winding will carry twice as much current as before. Maybe small step-up transformer would be a better option after all.