Digital fiber optic data link terminal modules are being developed in a number of laboratories, and often the modules take the form of black boxes without convenient access to internal signals. This paper discusses types of measurements which can be made using only the digital terminals of the link. Bit-error-rate (BER) vs signal power data can, of course, provide a means for proving overall link performance, but it can also be used to determine certain internal parameters of the receiver. The theoretical dependence of the BER vs light signal power curve shape on receiver input noise equivalent power and comparator slicing level is discussed. BER vs light signal power measurements obtained with an experimental link are presented and interpreted with the aid of the theoretical curves. The effect of other problems, such as hum or oscillation on the BER data, are mentioned. Sensitivity of the system BER to bit rate is also discussed, with application to determining system operating limits. Since BER measurements are simple to make and can easily be automated, they may prove to be a useful means for either laboratory evaluation or production-line acceptance testing of sealed terminal modules.