It is a very good question. Don't worry let me know about this all. Instagram doesn’t show exactly who shared your post, but you can check in a few ways which are given below:
For Feed Posts & Reels
Use Post Insights (Business/Creator Account Needed)
Switch to a Business or Creator account.
Open your post and tap View Insights below it.
Look for the share icon like this - () to see how many times it was shared (but not who shared it).
Check Mentions & DMs
If someone shares your post in their Story and tags you, you’ll get a DM or notification.
See Story Reshares (If Available)
Tap the three dots (⋮) on your post.
If it has been shared in Stories, you’ll see "View Story Reshares" (only for active Stories).
However, you can ask your followers to tag you when sharing. Or also post a story asking people to let you know if they share your content.