Measurement tools that can accurately locate and monitor congested Internet links would significantly help us understand how the Internet operates. However, developing such tools is challenging, especially when our concerned target is congestion on the core Internet links rather than that on the relatively easily measured access links. Congestion on core links - and persistent congestion in particular - can reveal systematic problems such as routing pathologies, poorly-engineered network policies, or non-cooperative inter-AS relationships. In this paper, we present Pong, a novel tool capable of accurately locating and monitoring a subset of non-access Internet links that exhibit persistent congestion over longer time scales. Pong takes advantage of the persistently congested link property to overcome the long-lasting challenges common for delay-based inference tools. In addition, it exploits the same property to (i) infer otherwise unknown underlying path conditions, (ii) determine appropriate queuing delay thresholds to reveal congestion, (iii) achieve high accuracy with low probing rate, and (iv) detect moments of its own inaccuracy. Finally, Pong can quantify measurement results' accuracy comprehensively, allowing us to further select vantage points that maximize the observability of the underlying congestion.