Actuation timing is an important parameter in powered ankle exoskeleton control that can significantly influence user experience and human-system performance. Previous studies have investigated the actuation timing through optimization under different objective functions, such as minimizing metabolic cost. However, little is known about people's psychological sense of actuation timing. This pilot study measured two subjects' sensitivity to small changes in actuation timing during walking. The just-noticeable difference (JND) threshold was determined via a fitted psychometric function, which quantified subjects' performance in discriminating between a pair of actuation timings. Subjects could detect changes of 3.6% and 6.8% stride period in actuation timing respectively, showing the difference in perception between individuals. The results from this pilot study provide a preliminary understanding of human perception towards exoskeleton control parameters, which offers insight on individual differences in exoskeleton usage and informs exoskeleton precision requirements to minimize undesired human-system interaction.