Two crucial claims of relativity theory can be directly confirmed. Utilizing elementary particles in high-energy accelerators, we can demonstrate that at high velocities objects are subject to time dilation and an increase in mass. However, a third claim of the theory, the Lorentz contraction thesis, which is connected to the other two, is not directly confirmable. But the fact that the theory in general is supported by experimental results indirectly confirms the contraction thesis.
The principle underlying the reasoning above is that a thesis that is related to other theses can be indirectly confirmed by the direct verification of the others.