System.Web.HttpBrowserCapabilities myBrowserCaps = Request.Browser;
        if (((System.Web.Configuration.HttpCapabilitiesBase)myBrowserCaps).IsMobileDevice)
        {
            ...        }
        else
        {
            ...        }

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
       
        

        Dim myBrowserCaps As System.Web.HttpBrowserCapabilities = Request.Browser
                
        If (CType(myBrowserCaps, System.Web.Configuration.HttpCapabilitiesBase)).IsMobileDevice Then
            Response.Redirect("Mobile_Index.aspx")
        Else
            'labelText = "Browser is not a mobile device."
        End If
end sub

Dim s As String
        s = UCase(Request.UserAgent)
        
        Dim n As Integer = 0
        For x = 1 To Len(s) - 6
            If Mid(s, x, 6) = "MOBILE" Then n = 1
        Next
        
        If n = 1 Then
            Response.Redirect("Mobile_Index.aspx")
        End If
        
        
        Dim myBrowserCaps As System.Web.HttpBrowserCapabilities = Request.Browser
                
        If (CType(myBrowserCaps, System.Web.Configuration.HttpCapabilitiesBase)).IsMobileDevice Then
            Response.Redirect("Mobile_Index.aspx")
        End If