How do I know when someone has shared my link on social media

On my website I want to make certain features available after a person has shared my link on Facebook or Twitter, etc. So the idea is to ask the visitor to share my link. Then after they share the link, to create a cookie for them. Then I would check said cookie to determine if they have access.

My question is how do I know the person has shared? So say the person clicks on the Facebook button. Then possibly he has to login into Facebook. After the actual sharing is done, what kind of feedback do I get so that I can go create the cookie?

If the whole thing can be done in javascript and html that would be great, but in case the server has to be involved, I am using python. Again, I know how to write my code about setting cookie and about asking someone to share on social network. The one thing I don't know is how do I get the feedback that the sharing actually took place?