A non-intrusive method of determining whether a mobile device is transmitting through a Wi-Fi router or a cellular network is described. The method includes monitoring messages relating to a wireless network. A subset of the messages including Create Session Request, and IMSI/MDN are received and later a subset of the messages including Delete Session Request, and the IMSI/MDN are received. The combination of the sets of received messages is used to determine whether the mobile device is transmitting through the Wi-Fi router or the cellular network, specifically based on the Create Session Request and the Delete Session Request.