If your user navigates to my web page how do i programmatically determine they're being able to access my page from inside the Intranet? (Not thinking about NTLM or Home windows forms authentication. Require a programmatic method.)

Read the Ip from the Http request after which make sure that address from the range that's the neighborhood intranet.

Think about these two clips for acquiring the client's Ip:

HttpContext.Current.Request.ServerVariables["REMOTE_ADDR"]

or

//this can be a wrapper for REMOTE_ADDR:

HttpContext.Current.Request.UserHostAddress

Whether it falls within 10.x.x.x, or 192.168.x.x, you already know your visit is from inside.

Otherwise, you can look into the URL for any known good server title or alias:

HttpContext.Current.Request.ServerVariables["SERVER_Title"]