The present disclosure is directed to a wearable electronic device, such as a watch, that includes one or more optical sensors. In order to determine accuracy of measurements by the optical sensors, the device detects whether or not the optical sensors are in physical contact with the user's skin. The device detects a level of contact between the user's skin and the optical sensors based on electrostatic charge variation measurements, and generates a contact reliability index (CRI) based on the level of contact. Operation of the optical sensors are adjusted based on the CRI.