Instagram is not that specific to show you exactly who shared your post while it does allow you to see the number of people who shared it. Basically, you have to switch to a Professional or Creator account, open the post, and tap on View Insights if you want to check the number of times your post has been shared. The shares are indicated by the arrow icon in the Insights, and it only shows the number of times the content was shared, but it does not disclose the usernames.