Since this is posted under the "steem" tag, I'm gonna go ahead and assume that you are talking about followers on Steem.
There are two tools I know that check the inactive~ness of the people following you:
SteemAgency - https://steem.agency/follower/followers
True Follower here means: an account who have posted at least once the past month. If you want, you can also filter out what constitutes as "Dead Follower" to you, either an account who had made at least one post/comment or one vote the past month.
If you scroll down, you will see a list of all your Inactive/Dead followers for easy unfollowings.
The other tool is SteemSpectacles, but at the moment it seems to be down and/or possibly on maintenance.