The gas particles in the disk will rub against each and heat up from the friction. The amount of friction increases inward causing increasing temperature closer to the event horizon. The disk will produce a wide spectrum of radiation. Close to the event horizon, the disk will be hot enough to emit X-rays. X-ray sources in the Galaxy are rare.
Stars are bright in the optical band.
A very small percentage of them can also put out significant amounts of ultraviolet radiation, but all stars are faint in the X-ray band. If you find an X-ray source, then you know something strange is happening with the object. If the unseen companion is very small, then the X-ray brightness of the disk will be able to change rapidly. The fluctuation timescale gives us the maximum possible diameter of the object. Since the speed of light is finite, it takes a given amount of time for light to travel across the object.