Bat droppings will be pellet-shaped, usually about a quarter of an inch long, uniformly dark brown, grey or black. If your bat house has an open bottom, you could occasionally shine a flashlight in to see them. At first, do this only very briefly and quietly to avoid frightening them into leaving. Thus far, most occupied bat houses have been placed between 10 and 25 feet above the ground, facing east or southeast. If one is occupied, others in the same area seem more likely to succeed.
Early reports indicate that the probability of occupancy varies greatly from area to area.
In some places 10 or more houses are still vacant after three years, while several members who have put up two or three houses have had all occupied within one to two years. The reasons for such variance are many and may not be due solely to placement. Environmental pollution and overuse of pesticides, inadequate surrounding habitat and a lack of required hibernation sites may preclude use in some areas. But in other cases, the local bats may not be the type that roosts in man-made structures, or they may have requirements not met in the bat house provided. In general, bat houses are most likely to succeed where bats already are known to roost in buildings or bridge crevices, especially where such options are being lost.