When a person unfollows you, you will notice that your follower count has decreased. Memorize the number of followers you have. and if you found it going down, know that someone unfollowed you. This can also happen for other reasons. See why my followers Instagram dropped.
If you checked your follower list one by one and you didn't find a person who was previously there, know that this person has unfollowed you. See why do people unfollow others.
When a person unfollows you, they will stop seeing your posts and so they will stop interacting with them. If all of a sudden someone stopped liking your posts, you might want to check if they are still following you.
There are many programs and apps that monitor your followers and notify you when someone unfollows you. Those tools will need access to your Instagram account and they will only record the new followers lost starting from the moment you started using them.
Visit the profile of the person who might have unfollowed you then check the list of people they are following. If you found yourself there, know that they didn't unfollow you. See why Instagram is so fake.
You will hardly know who unfollowed you without keeping track of the people who followed you in the first place. Unless you are using a special tracking software, you won't be able to know who unfollowed you unless you keep track of your followers.
Instargam will notify you if someone who unfollowed decided to follow you again. So if you got a follow notification from a person who was already a follower, know that they either clicked unfollow by mistake or that they unfollowed you intentionally.