No, often there is not enough evidence either way.
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There is no experiment or possible evidence that could prove that invisible snorgs do not exist. So the Snorg Hypothesis is not scientific. On the other hand, the "Negative Snorg Hypothesis" (that they do not exist) is scientific. You can disprove it by catching one.
The question is very poorly specified so this answer is simply a wild guess at what the questioner might want. Three possible outcomes of any research, designed to test some hypothesis, are: (a) evidence in support of the hypothesis; (b) evidence disproving the hypothesis; or (c) evidence that can neither prove (support) nor disprove the hypothesis.
The process is called hypothesis testing. If you are inquiring on how to prove or disprove a claim, search for the article on wikipedia.
The researcher's working theory of what s/he expects the research either to prove or disprove.
They are part of the scientific method. The test results confirm or disprove the hypothesis.