In order for a theory to be proved or disproved, empirical evidence must be collected. An empirical study will be performed using actual market data. For example, many empirical studies have been conducted on the capital asset pricing model (CAPM), and the results are slightly mixed.
In some analyses, the model does hold in real world situations, but most studies have disproved the model for projecting returns. Although the model is not completely valid, that is not to say there is no utility associated with using the CAPM. For instance, the CAPM is often used to estimate a company's weighted average cost of capital.