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.