Popper on Falsification

Just as chess might be defined by the rules proper to it, so empirical science may be defined by means of its methodological rules. In establishing these rules we may proceed systematically. First a supreme rule is laid down which serves as a kind of norm for deciding upon the remaining rules, and which is thus a rule of a higher type. It is the rule which says that the other rules of scientific procedure must be designed in such a way that they do not protect any statement in science against falsification.

Karl Popper, The Logic of Scientific Discovery (1959)
11. Rules as conventions., p. 54

Science has been predicated in large part on inductive reasoning. In short, observations are gathered and from these data generalizations are made. However, there are several problems with induction and Popper tried to sidestep these by predicating science instead upon deduction. This necessitates that scientific theories must in some way be testable. Hence, the hallmark of a scientific theory for Popper was its falsifiability. Strictly speaking, Popper's proposal still requires induction to some degree, but this induction is always checked deductively for its predictive ability.

So one key to determining whether or not a theory is scientific is to look at how it is worded. Certainly, if you see the hallmarks of inductive logic (eg. repetitive use of the word 'infer' but not the word 'predict') the theory may be suspect*. But more precisely, is the theory worded in such a way that it is testable? What is the crucial experiment that would falsify or corroborate this theory? Do that and you see how many logical systems, such as psychoanalysis, homeopathy, intelligent design, fall outside of anything Popper would have called science.

* Though that doesn't necessarily mean it is wrong.