End-to-end delay estimation is a crucial issue in the design of network monitoring systems for wireless best-effort infrastructures. This work demonstrates that estimations based on one-way delay are the most suitable for evaluating the delay on wireless networks such as Wi-Fi LANs and Bluetooth piconets. To prevent delay measurement from being affected by clock synchronization issues, raw measures need to be corrected by estimating clock synchronism attributes such as clock skew and offset. However, due to network noise and clock adjustments, this estimation process may affect the reliability of network monitoring systems. In this paper we propose a trustworthy network performance monitor designed to support adaptive and/or soft real-time applications in wireless environments. The monitor corrects one-way delay measurements through an on-line algorithm for evaluating clock synchronism attributes. Simulation experiments show the effectiveness of our approach.