How do we know?

The exit is in place. The holes are fairly sealed. Now to know when the bats have left (or if they have). I will do some watching in the evening at 'bat leave the house' time. But beyond that, I'm not sure how I decide that they are out.

Any good ideas?

Time to redo about 8 feet of ridge cap shingles on the main house. In the process, I'll add some much needed venting.

-EJK