We present an algorithm for detecting service provider alliances. To perform this, we modelize a cooperative game-theoretic model for competitor service providers. A choreography (a peer-to-peer service composition model) needs a set of services to fulfill its requirements. Users must choose, for each requirement, which service providers will be used to enact the choreography at lowest cost. Due to the lack of centralization, vendors can form alliances to control the market. We propose a novel algorithm capable of detecting alliances among service providers, based on our findings showing that this game has an empty core, but a non-empty bargaining set.