By every possible means, presumably. Okay, seriously, she thinks we can test such a theory in principle, but…
But what you really wanted to know, I guess, is whether these tests are practically possible any time soon? I do think it is realistically possible that we will be able to see these deviations from general relativity in the next 50 years or so. About the other tests that rely on models for the early universe or symmetry violations, I’m not so sure, because for these it is again possible to move the predictions and then claim that we need bigger and better experiments to see them.
Is there any good reason to think that such a theory of everything is correct in the first place? No. There is good reason to think that we need a theory of quantum gravity, because without that the current theories are just inconsistent. But there is no reason to think that the forces of the standard model have to be unified, or that all the forces ultimately derive from one common explanation. It would be nice, but maybe that’s just not how the universe works.Sabine Hossenfelder, “How can we test a Theory of Everything?” at BackRe(Action
See also: Slapping Sabine Hossenfelder Isn’t Going To Solve Physics’s Problems
Sabine Hossenfelder: There is a crisis in physics and it may spread to other sciences