A ‘theory’ is a an explanation for past phenomena that also predicts future phenomena. A good theory will even predict unexpected or surprising future phenomena. So to test a theory, all you have to do, in principle, is to derive a prediction(s) from the theory and then observe whether it is so in reality.
Of course there are snags, such as being biased or primed to looking for supporting or contradictory instances This is what research design is for. To protect us against conflicts of interests etc. This has nothing to do with ‘qualititative’ or any other kind of research over any other. The nature of the prediction will and should determine what evidence is needed to test it. Think about what kind and scale of evidence would convince someone else (maybe me) that the theory had been (so far) confirmed or shown to be false. Ditch the q words and focus on finding stuff out!
Have a look at Chapter Three in:
Gorard, S. (2013) Research Design: Robust approaches for the social sciences, London: SAGE, ISBN 978-1446249024, 218 pages