Emulation has become an excellent approach for the validation and evaluation of network research. It provides researchers with a contained, customizable, and scalable testing environment, which can be easily packaged and published for potential readers to reproduce its results. However, as the network components are only virtual, it lacks the inherent realism of physical testbeds. In light of this, monitoring specific metrics of the emulated network has been proposed as a solution to mitigate to some degree inaccuracies caused by emulation. While this is not difficult to implement in single-machine settings (e.g. with Mininet), in scenarios where the emulation is distributed over multiple physical machines (e.g. Distrinet) any monitoring is limited by their asynchrony. In this paper we tackle the case of packet delay monitoring, to which we propose a methodology for passively measuring delays with or without underlying assumptions about time synchronization. We implement and evaluate our proposed methodology in an open testbed and show that it can reach results within few microseconds of perfect accuracy.