Described herein are methods and systems for picking up, transporting, and lowering a payload coupled to a tether of a winch system arranged on an unmanned aerial vehicle (UAV). For example, the winch system may include a motor for winding and unwinding the tether from a spool, and the UAV's control system may operate the motor to lower the tether toward the ground so a payload may be attached to the tether. The control system may monitor an electric current supplied to the motor to determine whether the payload has been attached to the tether. In another example, when lowering a payload, the control system may monitor the motor current to determine that the payload has reached the ground and responsively operate the motor to detach the payload from the tether. The control system may then monitor the motor current to determine whether the payload has detached from the tether.