In short, a hypothesis is testable if there is a possibility of deciding whether it is true or false based on experimentation by anyone. This allows to decide whether a theory can be supported or refuted by data. However, the interpretation of experimental data may be also inconclusive or uncertain.
... the question of whether the auxiliary assumption is testable or not is not so easy to determine as it might first appear. Criteria regarding independent testability do not seem to be absolute. ... The least common denominator for all sciences is that hypotheses are formulated and tested. This is meaningful only if one is prepared to change one's mind after testing, to admit that even one's favourite hypothesis was wrong. ... The result of the test is either that the predictions and observation reports are compatible, or that they conflict. In the former case one may be justified to say that one's hypothesis is supported. In the latter case one must reconsider something; one must reject either the hypothesis, some auxiliary assumption, or the observation report.
The purpose of each test you run is to see what happens when you intentionally alter your usual conduct and then reflect upon the meaning of the results for your big assumption. ... To make [the assumption] testable, you may have to back up and unearth a prior assumption in the sequence ... Once you've chosen a big assumption to test, the next step is to design your first experiment to challenge it.Robert Kegan and Lisa Lahey suggest how to turn personal tacit assumptions into explicit testable hypotheses in everyday life.
Consequently, the universal statements, which are contradicted by the basic statements, are not strictly refutable. Like singular statements and probability statements, they are empirically testable, but their tests do not have certain, definite results, do not result in strict verification or falsification but only in temporary acceptance or rejection.
Theories may be more, or less, severely testable; that is to say, more, or less, easily falsifiable. The degree of their testability is of significance for the selection of theories. In this chapter, I shall compare the various degrees of testability or falsifiability of theories through comparing the classes of their potential falsifiers. This investigation is quite independent of the question whether or not it is possible to distinguish in an absolute sense between falsifiable and non-falsifiable theories. Indeed one might say of the present chapter that it 'relativizes' the requirement of falsifiability by showing falsifiability to be a matter of degree.
The idea that some experiments really do test a proposition, while others do not, is not controversial, nor does it deserve to be. ... Testing is to testability as dissolving is to solubility. If we can understand what testing is, we also should be able to understand what testability is.