Actuator

Every joint of a robot has an actuator, so a humanoid carries dozens of them. Their quality sets how strong, quick, quiet and precise the robot is, and they are the most expensive part of the machine.

Look at peak torque, speed, weight and whether the unit is sealed against dust. Actuators are also what wears out first, so ask how long they last and whether a single joint can be replaced without sending the whole robot back.