desde un iframe sharepoint detecte el userid

Hello,
I would like help to know how I can check the user id when I access the sharepoint, I do it through a .net application (executed directly from the web I have no problems) but when said app I put it inside an iframe so that it comes out inside a sharepoint site, here comes the problem since it does not detect the userid.
Is there a way to detect the sharepoint userid from a sharepoint iframe?
Thanks.