While his analysis of what scientists do is basically wrong (an idealisation which scientists tend to believe, but doesn't really reflect what they do), his definition of what a scientific statement is and isn't is a good rule of thumb. He also showed that historicism and marxism are not scientific. He also demolished psychoanalysis and sociology.
I haven't got any of his books to hand unfortunately, so this is from memory. However he basically said that if a statement isn't falsifiable, then it isn't scientific.
So, Einstein's special law of relativity is scientific because it's predictions can be falsifiable by observation/measurement. It stands as a theory because it has not been falsifiable. Not because it uses complicated mathematics, or sounds scientific, or is in an area that is deemed scientific. A scientific statement can never be proven, and it's power of prediction is dependant upon the observations/experiments that support the theory.
Similarily Marxism is not scientific because it uses dialectics (thanks to all the people who explained that. I've come across the concepts elsewhere. There's good stuff on that in chaos theory/complexity theory; as well in certain areas of formal logic. The tools have improved somewhat since Marx's day) - any more than economics is because it uses complicated algebra. Because the statements of Marx could not be falsifiable, Marxism is not scientific.
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