Well basically, the idea is obviously we cannot see a blackhole. But, we know where they are, because they bring about the brightest sources of light in the universe, which is a little ironic. Especially in the case of super massive black holes that are in the center of galaxies. Being so big, while they don't eat things very well, their gravitational pull makes things around it spin very fast. Since they are around a lot of gas, this gas spins up, becomes excited and ionized, and give off a lot of light. That accretion disk of gas and the light it radiates is a quasar.