A system for detecting an optical network condition, such as a fiber bend, has an optical line terminal (OLT) that is configured to communicate control information with optical network terminals (ONTs) indicating the transmit power levels and the receive power levels of optical signals, such as optical data signals, that are communicated between the OLT and the ONTs. Based on such information, line losses at different wavelengths are determined and then compared in order to detect an optical network condition, such as a fiber bend. Since the measurements can be performed on optical data signals ordinarily communicated between the OLT and the ONTs, the testing may be performed during data communication. Further, as long as optical communication between the OLT and the ONTs is possible, a fiber bend or other network condition may be detected at any point along the optical path.