If user visits my website from a desktop then an hour later visits the site from her cell phone, I need to track to see it was the same user. One way to do this is I need to somehow anonymously read the unique id that is attached to the google profile of the signed in user and pass it to my google tag so Google Universal Analytics can track user sessions across multiple platforms.
Can you please suggest the best way to do this, I might even not need to access profile id for this as I know this might be a little hacky.
Users will not sign into my website so I can't use my own authentication mechanism.