The key is the mass of the compact object . We know from our studies of stellar evolution that if we have less than 1.4 solar masses in the stellar core, the result will be a white dwarf. So, remembering back when we derived the mass of stars in binary systems, if we can measure the period of the binary system, and the separation between the stars, or infer those, we can get the mass of the compact object. There are many kinds of black hole binaries. For the purpose of this class, we will divide them into three types .
Some black holes have a large star such as an O or B star as the companion.
Such large stars have a powerful stellar wind, and the black hole accretes gas from the wind . Some black holes have a small star such as a K or M star as a companion. In this case, the small star may have evolved to a giant, and when it becomes big, some of the gas will feel a stronger gravitational force from the bH than from its own star, and that gas will be pulled off of the companion onto the bH . It has two powerful jets shooting out gas with speeds of the speed of light. Those jets precess around like a lawn sprinkler, leading to many interesting effects.