In this work we address the problem of inferring the presence of a bottleneck from passive measurement in the UMTS core network. The study is based on one month of packet traces collected in the core network of mobilkom austria AG & Co KG, the leading mobile telecommunications provider in Austria, EU. During the measurement period a bottleneck link in the UMTS core network was revealed and removed, therefore the traces enable the accurate analysis and comparison of the traffic behavior in the two network conditions. The proposed approach exploits statistics of estimated TCP performance parameters (e.g. RTT, re-transmissions) in order to build a set of bottleneck indicators. We show that such statistics are volatile due to the presence of few top users, but this effect can be counteracted with a simple filtering method. Results show that the frequency of re-transmission events is a powerful indicator for the specific type of bottleneck under study, and it can be used to provide early warning about future occurrences of similar events. This application is particularly important for operational UMTS networks nowadays, since the traffic volumes and composition is still under evolution.