Check whether a live id user is signed in ??

im creating a website for Goalive Challenge for Microsoft.
How doi check whether a "live id " user is signed in or not. if the user is not signed in i want to redirect to another page.
Could you help me?
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when i tried to integrate with asp.net membership it shows an error
protected void btnRegister_Click(object sender, EventArgs e)
        {

            MembershipUser user = Membership.CreateUser(UserId, UserId, txtEmail.Text);
            //MembershipUser user = Membership.CreateUser(UserId,txtUserName, txtPassword);
            if(user!=null)
            {
         &nbs ...