There are many factors to determine legitamacy of followers. The first steps is to look at the engagement rate. Ther average Instagram user has an engagement rate of 3%, although this number can go down as followers increase. By looking at your reports, you can find average engagement rates based on follower count.
Second, you should look at the account history. Usually accounts with fake followers will purchase one-time and their followers will rarely increase. In fact, in many cases they will decrease. Take a look at the account history report to view how many followers any account is receiving (or losing) on a daily basis.
Third, check the comments. Although the numbers can look good on the surface, you want to check this manually to be 100% sure. Are the comments related to the picture? Are other users tagging their friends? If you’re seeing a large number of “cool pic” type of comments, most likely these are fabricated, or their following is largely from bots.