No experiment that could test what is called string theory is known of. So, it is still a speculation, not properly a theory. But it is an extraordinarily detailed speculation. Progress is measured by discovering details that imply or contradict current theory, relying on the "correspondence principle".

If tests for it are ever devised, and it survives, it will be a "Grand Unified Theory" that encompasses general relativity and quantum mechanics, the way those do Newtonian mechanics. Theory is the end state, not the beginning.

In between speculation and theory is hypothesis, a treatment of actual evidence that does not fit into, or anyway is not covered by, the currently favored theory, which has come to be called the Standard Model. This has happened in quantum physics many times, and the theory ends up altered to fit, getting a little more complicated each time. It has lately been hard to find anything new that the Standard Model doesn't get right. Lately, people complain that its numbers are arbitrary and not esthetically pleasing.

But the Standard Model has long been known to be incompatible with General Relativity. So far we have not found cracks in either one. Chances are it will be centuries before we could properly test a grand synthesis, if we could invent one. "String theory" might hold the outlines of such a synthesis, or might be just a mathematical oddity with curiously physical qualities, a model of a possible universe that does not happen to match ours. Even our great-grandchildren will most likely never know.