Systems and methods determining measurement confidence for data collected from sensors of a wearable device are herein disclosed. In an implementation, a confidence measurement that a wearable device is worn by a user can be determined by determining that the wearable device is not in motion, comparing sample voltages collected using a light emitter to thresholds indicative of a surface on which the light is being reflected, calculating a signal quality metric using data collected using a pulse oximeter, and comparing the signal quality metric to thresholds indicative of typical biometric data measurements. Other implementations for confidence measurement can include frequency transforming signal data stored in a buffer, performing probabilistic modelling on the frequency transformed data, and determining a confidence measurement using a signal quality estimation based on the modelled data.