How to test the backend of GUI progams ?

The back-end of my GUI program is about 50% complete. It is already so "huge and complex" (already) that it cannot be tested using a console interface. That would involve programming too many complicated loops and such, which is a pain. I did test some small modules while coding, but I doubt if that will suffice. I guess that I will have to develop GUI along the way to test every feature after i create it. Is this the best way or is there a better way to test my code ?

To be honest, if you can't test the application without a GUI you have integrated too much of the application logic into the GUI code. It would have been better to have separate "engine" code that just gets triggered by the GUI.