You cannot see who sent your Instagram post to someone else
Instagram does not show a list of people who shared your post privately
You can only see share insights if you have a professional account
Open your post
Tap View Insights
Check the share count under the insights
Use Instagram Story mentions or tags to see who shared your post to their story
Check your DM requests and message threads for reposts or shares sent directly to you
If someone reposted your content, check the account that posted it
If you want to know who sent it, ask the recipient directly