The amount of 'perihelion shift' each century was well known at the time Einstein provided a complete explanation for it in 1915. Light at every frequency can be bent in exactly the same way by gravity. Clocks run slower in strong gravitational fields. This was confirmed by Robert Pound and George Rebka at Harvard University in 1959, and by Robert Vessot in the 1960's and 70's using high-precession hydrogen maser clocks flown on jet planes and on satellites. Gravitational mass and inertial mass are identical.
Black holes exist.
Although these objects have been suspected to exist since they were first introduced to astronomers in the early 1970's, it is only in 1992 that a critical acceptance threshold was crossed in the astronomical community. It was then that Hubble Space Telescope observations revealed monstrous, billion-sun black holes in the cores of nearby galaxies such as Messier 87, Messier 33 and NGC 4261. Gravity has its own form of radiation which can carry energy. Russel Hulse and Joseph Taylor in 1975 discovered two pulsars orbiting each other, and through careful monitoring of their precise pulses during the next 20 years, confirmed that the system is loosing energy at a rate within 1 percent of the prediction by GR based on the emission of gravitational radiation. A new force exists called 'gravito-magnetism'.