Abstract : In recent years network neutrality has been widely debated from both technical and economic points of view. Its proponents advocate that all network traffic should be treated equally by Internet Service Providers (ISP's) and no discrimination should take place on origin, destination, content or load. Opponents on the other hand maintain that the components that constitute the Internet of today already apply some forms of preferential treatment at different levels, and it would be neither feasible nor desirable to enforce a purely egalitarian principle, also in light of emerging user needs. Nevertheless various cases of traffic differentiation at the Internet access have been reported throughout the last decade, in particular aimed at bandwidth consuming traffic flows and alternative competing services. In this paper we present a novel method for the detection of traffic differentiation, through which we are able to correctly identify where a shaper is located with respect to the user and evaluate whether it affected delays, packet losses or both. The tool we propose, ChkDiff, reuses the user's own traffic and replays it in order to target routers at the first few hops from the user. By comparing the resulting flow delays to the same router against one other, and analysing the behaviour on the immediate router topology spawning from the user end-point, ChkDiff manages to detect instances of traffic shaping and accurately locate them. We provide a detailed description of the design of the tool for the case of upstream traffic, the technical issues it overcomes and a validation in a controlled scenario.