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.
Tag them all in a post, and ask them to verify that they are real, that is one way, but very unreliable.
Here is the truth of the matter. Many bots auto follow accounts, many accounts have gone inactive, many accounts have been sold, many accounts have been lost due to improper storage of keys, many accounts are just antisocial creatures lurking the blockchain.
Probably 10-20 percent of your followers are actually live human beings, still using their accounts. This might even be a generous estimation.
If you were to create a post, and tag all of your followers in the post, you might be surprised when you get less than 10 comments, maybe even less than 5. Much of this is dependent on your level of interactions with your followers, and what kind of followers you've attracted over time.
There is no perfect answer to this, but honestly, more of your followers are probably dead than you may realize.
There are a few sites that you can go and double check.
Steemspectacles is the one I use as it shows you which users have commented on or engaged with you over the last 100 days. It will show you active followers ,dead followers and ghost followers.
The other site is Steemagency https://steem.agency/follower/followers and it will give you the details similar to steemspectacles.
Don't be alarmed by how many dead followers you have as we all have them and a good guide is if you have 1000 followers you will find between 10 and 20 % are genuine and alive. The other 80% are dead or have left the platform and will be back hen the Steem price rises again.
It depends on the platform in question. If it is twitter, you can use the twitter audit. If it is steemit, I have a zilch idea of how you can do that. When someone gives a useful answer, please don't hesitate to tag me.