The current techniques of optical system performance monitoring are described and characterized. From the available techniques, we focus our attention on the asynchronous Q method, and it is described and evaluated for several working conditions. This technique allows optical performance monitoring without synchronization, thus allowing the performance monitor to work at a much lower ratio and without performing clock recovery/synchronization, which is quite a hard and expensive task to perform, especially if considering different bit rates and pulse formats. The results of this technique are discussed, taking into account linear and nonlinear effects. Its resilience to the pulse format is also observed by considering a square pulse, where the "on" time is much more evident than the transition time, and a raised cosine pulse where the conditions are reversed. Some experimental results are shown and validated with the synchronous BER testing standard technique.