As PONs carry increasing amounts of data, issues relating to their protection and maintenance are becoming crucial. In-service monitoring of the PON's fiber infrastructure is a powerful enabling tool to those ends, and a number of techniques have been proposed, some of them based on optical time-domain reflectometry. In this work we address the required features of PON monitoring techniques and review the major candidate technologies. We highlight some of the limitations of standard and adapted OTDR techniques as well as non-OTDR schemes. Among the proposed optical-layer monitoring schemes, we describe our novel optical-coding-based reflection monitoring proposal and report on recent progress. We end with a discussion of promising solution paths.