The adequacy of an explanation tells us that if the hypothesis is true, the explanandum has to be true as well. It tells us, in other words, that we may accept the hypothetical proposition: if H, then E. And we know that E is true. But to infer from these two propositions that H is true would be to affirm the consequent--and that's a fallacy. There might well be some alternative hypothesis H' that would explain E with equal adequacy.
The canons of scientific research require that a theory not be accepted until it has proven its superiority to rival theories.
Assuming, then, that we have several plausible hypotheses on the table, how do we decide which one is true? For some hypotheses, the standard methods of observation and induction can be used. However, in other cases, an indirect approach must be used.