Constructor theory is a ‘theory of principles’. A principle in this sense is a law about laws, not directly about physical objects. Familiar examples in current physics are the principle of equivalence in relativity, the principle of locality, and the principle of conservation of mass in pre-relativistic physics.
Testing a theory always needs there to be at least one rival theory, otherwise the best explanation for the failure of a test is simply ‘experimental error’ etc. When there are rival theories making different predictions about measurable quantities, a crucial test can be performed and the ones making false predictions can be tentatively discarded.
Testing a principle is similar, but at one step removed: again at least two rival principles are needed. The way one tests them is to first deduce predictions from rival laws conforming to each of them. Then, one tests that prediction. For example: the principle of energy conservation has important implications for models describing a skier sliding down a mountain slope. One can then take two models that describe the skier’s motion: one, e.g. Newton’s laws, obeys the principle of conservation of energy. The other, does not: for instance, it predicts that the skier ends up having more kinetic energy at the bottom of the slope than it had potential energy at the top. Then, one performs an experiment with an actual skier, to test one model against the other. So far, all experiments testing models that predict spontaneous creation of energy have been refuted.
Principles in constructor theory are tested in the same way. For instance: the principle of interoperability of information requires that the composite system of two information media is also an information medium. This implies that certain transformations are possible on the composite system, irrespective of the details of their dynamical laws. As in the previous example, for the test we need two rival models describing the composite system of two information media. One model could predict that the principle is not obeyed, for instance because the two systems can only imperfectly interact with each other, in a way that for example perfect copy-like operations are not allowed. Confirming the prediction of one or the other model would test the interoperability principle of information. The same holds for the other constructor-theoretic principles.