Usually a theory is thought to be "proper" if no evidence is found, that she's not true. Its called "Falsifiability".
Thats a bit vice verse but done, because you can never can set up tests for all circumstances to prove the truth of a theory.
Anyway, if you find one point, where the theory does not work, its usually broken, at least for this topic.
It was a kind of enlightment in science history to step away from the believe in the absolute truth to the believe in the chance of truth
Refering to General Relativity, it has passed ALL falsification tests successfully so far.
Thats why it is considered to be a pretty good
theory. Though that does not mean, that there is an experiment someday, that reveals a weakness.
But until then, its status is "clean".
Just keep in mind one central aspect, which perhaps becomes visible in my last document link.
A theory should have content, which is actually useful to predict or describe some aspect of physics/nature. The way it works, the extensiveness etc. is a secondary aspect.