Bats are creatures of the night and hunt/fly about in search of insects for nourishment. During the day they rest or sleep. I you suspect bats are rousting in your attic investigate the attic for bat droppings. If bats are rousting in your attic you will certainly have bat droppings. You should investigate your attic to make sure no bathroom or kitchen vents are venting into the attic rather than to venting outdoors. The moist vapor from these vents attracts the bats and makes it a comfortable environment for them to roust.
Bats hunt at night so when they are out hunting you should seal all the entry points they are using to set up their rousting area. Screen out these entry points in your attic to prevent the bat access.
When bats are breeding and feeding their young in the spring-early summer don’t block the entrance yet. If you do you may block the females from returning to nurse their young, and the young will slowly starve to death. This I find in-humane and also a loss whereas the bats eat their own weight every night in mosquitos which helps human problems. Just go outside just before dusk keep an eye on your roof to see the entry/exit point of the bats. After the breeding season late summer early fall seal up these entrances after the bats leave to hunt and to prevent the bats returning. Use screen or caulk the openings.
Because bats are so important to mosquito eradication you may want to build a ‘bat house’ on the side of your house to give them shelter to roust and retain their insect-consumption services. There are plans for bat houses available on the Internet, and some places that sell bird houses also carry them.
If you don’t want to have the trouble of getting rid of the bat yourself, you can hire a bat removal company to do the work for you..