Well, it is a good feature of a theory. Imagine that you would have two competing theories but they give different predictions, then a measurement on those pred

Well, it is a good feature of a theory. Imagine that you would have two competing theories but they give different predictions, then a measurement on those predictions would determine which is which. (If they give the same predictions, an interesting questions would be 'is there a mathematical proof that they are the same?'.) For example with GR, it explained for example the precession of Mercury's elliptical orbit, and predicted time dilation (used now in GPS for example) and bending of light. It is true that gravity waves remains to be directly detected. This is how physics have usually progressed in the past: Have an old theory, have some experiments not explained by that theory, invent new theory that can be reduced to the old theory and explains these new experiments and give new predictions. (The subject of string theory is somewhat different however.)