It has been tested with sub-atomic particles and close binary stars. In both cases the mass change has been observed and agrees with Einstein's predictions. The length of an object decreases with its velocity. Again, Einstein's equations predict that an object's length would become zero at the speed of light. This has been tested indirectly by an experiment depending on something called the Mossbauer Effect.
Time slows down for a body that is moving and for one in a gravitational field.
This is one of Einstein's most fascinating predictions. Again, it has been tested many times. Sub-atomic particles last longer before decaying when they are moving close to the speed of light. Using Atomic clocks it is possible now to measure these effects on the Earth. At the speed of light all time slows down to zero.