How do I find my already installed air tag

Chattanoogan is right. Also, you can play a sound and trace the Airtag.

Play a sound

1. Open the Find My app on your iPhone, iPad, or iPod touch, then tap the Items tab.
2. In the Items list, choose the AirTag that you want to play a sound.
3. Tap Play Sound. If your AirTag is within Bluetooth range, it will play a sound.
4. To stop playing this sound before it ends automatically, tap Stop Sound.

Find your keys, backpack, and more with AirTag - Apple Support

By using FindMy on one of the trusted devices on the same AppleID.

AirTags are displayed in FindMy’s “Items” tab.

If it is under somebody else's Apple ID then you cannot use it.