title: How to know the user is from site A
body: <p>I have certain API for site B, but i need to let site B know when is the right time to execute them. The right time is when user come from site A. How can i make sure the visitor is from site A? the programming language of site B maybe C# aspx, or php. </p>
  
  <p><strong>update</strong>
  I need to make the entire site B know about user from site A. or else new page or redirection would cause the script not working. </p>