Well I am not really sure on how developed people think it is, but for the experimental data, I am not sure if we will see any given that it deals with a fundamental particle on the Plank length. However, it has offered working solutions to problems where there were none and that is about as...
As far as what strings are made of I understand that in string theory they are fundamental and not really composed of anything (verification from anyone?). I have no idea what actually makes them vibrate or chooses the pattern that they vibrate in, but there are some interesting ideas discussed...
I believe it can also be proved in a sense if it can accurately predict phenomena or produce answers to previously unanswerable questions, as far as I understand at least.