Emerging mobile devices are equipped with multiple interfaces allowing users to take advantage of heterogeneous radio technologies (e.g., cellular networks and wireless LANs). To support mobility and quality of service (QoS) for such multi-interface mobile devices, it is important to dynamically determine which interface or interfaces a mobile should use and when to switch from one interface to another. To make such decisions, it is critical for the mobile to compare the "qualities of the interfaces (QoI)", which may be measured by a variety of metrics, in real time so that the mobile can switch from one interface to another without user perceivable interruptions to on-going applications. This paper presents and analyzes new techniques for a mobile device to make rapid comparison of the quality of the interfaces using quickest sequential change detection and sequential two-sample hypotheses tests.