Science Buddies: "Ask an Expert"

Hi Experts,
Need help to figure out whether the sensor in working on the robot. Tried in a dark closet without. any source of light but Robot keeps moving.
What is the indicator to understand if the sensor is working.
Thanks for the help

What kind of robot is it? Maybe it has a self-diagnostic in the manual or listed online that would let you know if it's not working or just working but too sensitive? -ER