In Chapter 1: Huangling I - Act II, there will be a quest mission where you need to use a Sensor to confirm the patrol route to find the Detection Beacon.
Where to Find Sensor in Wuthering Waves
Assuming you are currently at the said quest 'Echoing Marche,' all you have to do is open the Utility tab by tapping Tab (for PC). There, you can see the eye icon on the top left. Click on it to equip the Sensor.
You should see the eye icon on your Utility tab now, beside your Resonance Skill. Just tap T and you should be able to progress through the quest.
If you're on Mobile, the Utility icon is on top of your Basic ATK. Hold onto the icon to change the utility you have and equip the Sensor. Then, you're good to go.
