String theory has tons of vacua, which make it hard if not impossible to make predictions of low-energy physics beyond what the Standard Model already predicts (like predicting electron mass or something like that). I think this is what most people mean when they say that 'string theory is not testable.'
But what about at high energies? If hypothetically we had a Planck-scale particle collider, does string theory make falsifiable predictions there?
Yes. The first being extra dimensions which we would be able to evaluate if true or not all the way to the planck scale.
The other would be supersymmetry, which while not specific to string theory, all current candidate models rely on supersymmetry to remove tachyons from the theory. Alternatively, we might see strong evidence of tachyon condensation (imaginary mass particles) which is a key process in spontaneous symmetry breaking and is hypothesized in string theory. Of note tachyon condensation is a process used in the Higgs mechanism so is already a part of practical theory after the Higgs Boson was observed.
One prediction that is a consequence of Maldecena's (link is to work by Hofman and Maldecena) work is the existence of an ultrastrong regime.
String theory predicts that, among the unique features of such forces, production of quark-like and gluon-like particles would lead to near-spherical distributions of many hadron-like objects, each with low motion-energy, and that jet-like sprays will not occur
Essentially, as we probed closer to the Planck scale we would see more and more exotic particles (or more frequently their decay products) which would behave differently than typical particles now observed. For reference here is discussion on two and three jet events.
This covers some of the more exciting possibilities.
Of further note, string theory is attractive because it is one of the few theories that actually can make predictions down to the planck scale and beyond.
