A wireless power system has a wireless power transmitting device and a wireless power receiving device. The wireless power transmitting device may be a wireless charging mat or other device with a charging surface. The wireless power receiving device may be a portable electronic device receiving transmitted wireless power signals from the wireless power transmitting device while resting on the charging surface. During digital ping operations, low-power wireless power signals may be transmitted from the wireless power transmitting device. Information gathered during the digital ping such as wireless power transmission efficiency information, coupling coefficient information, Q-factor information, and coil inductance information may be used in determining whether a foreign object is present. Control circuitry in the transmitting device can cause the receiving device to present an alert for a user indicating that wireless power transfer operations are commencing in response to determining that no foreign object is present.