This paper presents Eureka: a generic methodology of measuring the instantaneous (per second) bandwidth usage of networked games and applications in run time. Eureka starts with constructing a priority queue and sending low priority traffic through it. Then, the application under study is started with its packets marked as high priority. These packets therefore displace the previously existing low priority packets on the priority queue. We can measure this displacement and thus, the bandwidth used by the application. Unlike tcpdump this can be done while the application is running and without knowing any application specific data. In this paper we use Eureka to study the bandwidth usage of Massive Multiplayer Online Games (MMOGs), BZFlag and World of Warcraft-Trial, along with the 3D multi-site collaborative environments. A major strength of our methodology is that it provides run-time estimations of the bandwidth usage without interfering with the application, the environment, or the server/peer data. Moreover, Eureka is independent of the application and new infrastructure or tools are not required.