Sensor localization is one of the big problems when building large scale indoor sensor networks because GPS (Global Positioning System) is unavailable in indoor environments. We are developing a sensor localization system using WiFi APs as anchors, which requires no anchor deployment. Because sensor nodes cannot demodulate WiFi signals, we have developed a cross-technology RSS (received signal strength) measurement scheme with an AP recognition scheme [1, 2]. In this paper, we present WiChest, a WiFi-AP operating channel estimation scheme to accurately measure AP-RSS on sensor nodes. AWiFi channel overlaps with four ZigBee channels. AP-RSS depends on a ZigBee channel in which a sensor node measures the RSS. Using WiChest, a sensor node automatically switches its channel based on an estimated AP channel prior to RSS measurement. We implemented WiChest using a MICAz sensor node. The experimental evaluations reveal that WiChest accurately estimates AP operating channels with an F-measure of 0.80.