IEEE 802.15.4 is the technology behind Wireless Sensor Networks (WSNs) and ZigBee. Most of the IEEE 802.15.4 radios operate in the crowded 2.4GHz frequency band, which is used by many technologies. Since IEEE 802.15.4 is a low power technology, the avoidance of interference is vital to conserve energy and to extend the lifetime of devices. A lightweight classification algorithm is presented to detect the common external sources of interference in the 2.4GHz frequency band, namely, IEEE 802.11-based Wireless Local Area Networks (WLANs), Bluetooth, and microwave ovens. This lightweight algorithm uses the Energy Detection (ED) feature (the feature behind Received Signal Strength Indication (RSSI)) of an IEEE 802.15.4-compliant radio.