The scientific method is the best way yet discovered for winnowing the truth from lies and delusion. This leaves out the co-operation between scientists in building theories, and the fact that it is impossible for every scientist to independently do every experiment to confirm every theory. Because life is short, scientists have to trust other scientists. So a scientist who claims to have done an experiment and obtained certain results will usually be believed, and most people will not bother to repeat the experiment. Experiments do get repeated as part of other experiments.
Most scientific papers contain suggestions for other scientists to follow up.
Usually the first step in doing this is to repeat the earlier work. So if a theory is the starting point for a significant amount of work then the initial experiments will get replicated a number of times. This refers to the observed pattern of the slow extension of scientific knowledge with occasional sudden revolutions. This does happen, but it still follows the steps above. In popular usage, a theory is just a vague and fuzzy sort of fact.