Quantifying user behaviors at sit-stand desks is critical to understanding the efficacy of these workstations to reduce exposures to prolonged sitting or standing. A custom after-market desk-mounted sensor predicting sit-stand desk use by measuring desk floor heights, desk user distances, and the temperature in front of the desk was tested. The authors performed sensor component testing, a survey of desk height ranges, and a logistic model to estimate gross posture and user presence with a 95.6% accuracy rate. This approach can aid researchers and ergonomists in understanding sit-stand desk use in the field, with the end goal of reducing sedentary work activities.