All black holes, by their very nature, are invisible. But scientists hunting for supermassive black holes probe the centers of galaxies, looking for how the suspected monsters gravitationally influence the stars and dust near the cores. The black hole's gravity acts like a powerful lens, bending the light of a background star, so that it appears as two separate images when the black hole slowly drifts in front of it. However, the black hole's gravity also magnifies these stellar images, causing them to brighten as the black hole passes in front. Hubble's identification of the lensed star allowed for an accurate estimate of the mass of the black hole.
Careful analysis reveals that each black hole is approximately six times the mass of the Sun.
If they were ordinary stars with this bulk they would be bright enough to outshine the more distant background star. The masses are also too large to be white dwarfs or neutron stars. This leaves a black hole as the most likely explanation.