Gravitational waves are evidence, two extremely large objects merged, not that a black hole was formed.
EDIT: If our instruments were precise enough and we could remove the noise, we could theoretically detect gravitational waves of two Sun sized stars merging (2 solar masses shouldn't form a star capable of becoming black hole).
This is not correct, the nature of gravitational waves emitted by the merging of two objects is different in its final stage, depending whether the objects form a black hole or not. Until a black hole is formed, the nature of the emission is the same for all objects (e.g. black holes or neutron stars), but if a black hole forms, the final emission (ringdown) is different.
The first LIGO-Virgo events detected black hole mergers which resulted in black holes.
The last LIGO-Virgo event detected two neutron star mergers. Unfortunately the sensitivity of the apparatus is not good enough to determine if the merge resulted in a black hole, or a neutron star based purely on gravitational wave measurements. However, we have secondary evidence[1] that the remnant was a black hole.
But they can tell the distance between centers of mass and total mass. Haven't we observed that distance shrink until it reaches their Schwarzschild radius?
See my parallel comment, ringdown is different for black hole remnants vs. something else, so in principle we can tell if it resulted in a black hole or not. However, for objects of suitable mass (a.i. small) the current detectors don't work very well at the frequency required to make this assessment accurately, so secondary evidence is required.
EDIT: If our instruments were precise enough and we could remove the noise, we could theoretically detect gravitational waves of two Sun sized stars merging (2 solar masses shouldn't form a star capable of becoming black hole).