Sensor localization is one of the big problems when building large scale indoor sensor networks because GPS is unavailable in indoor environments. In this paper, we propose a sensor localization system using WiFi APs as anchors. WiFi APs are largely installed in indoor environments and are managed by a network system manager. Using WiFi APs as anchors, we can localize sensor nodes with neither newly deployed anchor nodes nor user cooperation. As a first step of our sensor localization system, this paper presents a WiFi AP-RSS monitoring system using sensor nodes. Sensor nodes are equipped with ZigBee (IEEE 802.15.4) modules, which cannot demodulate WiFi (IEEE 802.11) signals. We therefore developed a cross-Technology signal extraction scheme with an AP recognition scheme on sensor nodes. We herein describe the design and implementation of our AP-RSS monitoring system. The experimental evaluations demonstrated that the proposed AP-RSS monitoring system successfully recognized WiFi APs with detection errors less than 10%. We also confirmed that the proposed system monitored AP-RSS with an average error of 1.26dB.