We consider the problem of detecting and localizing of link quality degradations in transparent wavelength division multiplexing (WDM) networks. In particular, we consider the degradation of the optical signal-to-noise ratio (OSNR), which is a key parameter for link quality monitoring in WDM networks. With transparency in WDM networks, transmission lightpaths can bypass electronic processing at intermediate nodes. Accordingly, links cannot always be monitored by receivers at their end nodes. This paper proposes the use of optical multicast probes to monitor OSNR degradations on optical links. The proposed monitoring scheme consists of two steps. The first step is an off-line process to set up monitoring trees using integer linear programming (ILP). The set of monitoring trees is selected to guarantee that significant OSNR degradations can be identified on any link or links in the network. The second step uses optical performance monitors that are placed at the receivers identified in the first step. The information from these monitors is collected and input to the estimation algorithm to localize the degraded links. Numerical results indicate that the proposed monitoring algorithm is able to detect link degradations that cause significant OSNR changes. In addition, we demonstrate how the information obtained from monitoring can be used to detect a significant end-to-end OSNR degradation even though there is no significant OSNR degradation on individual links.