Typically people can hear them making scratching noise in the ceiling or wall void areas. Another indicator are bat droppings on the attic floor or in the insulation.  Once they have made their way inside bats are not often seen hanging in the rafters. More often bats prefer to hide in smaller confined spaces making them hard to locate. Oftentimes bat activity is not noticed until there is a bat flying around inside the living space. 

Bats can carry rabies and bat mites, fleas, and other insects and need to be addressed as soon as you realize you have an infestation.