How can I tell if someone has cloned my iPhone?

I have never heard of anybody outside Hollywood movies "cloning" an iPhone. If they did it in 20 seconds that would be classic Hollywood.

The person may have added you to their Contacts and set up a Share My Location connection.

Share your location in Find My on iPhone - Apple Support

To Airdrop, they would need to be on the exact same WiFi network that you are on. To show up in Find My, they would need to be using your same Apple ID, unless you are actively sharing your location. This would be something that you would have approved on your device.

Share your location in Find My on iPhone - Apple Support

Losing control of both your iPhone and your passcode would be Bad. Very Bad.

As for Find My, run Safety Check: How Safety Check on iPhone works to keep you safe - Apple Support

If they had your passcode, there are other risks, not the least of which are passwords exposed.

Some of that: Additional considerations when using Safety Check - Apple Support

Particularly if changes were made that allow anyone else to track you (without your knowledge and prior permission), this whole mess certainly at least approaches abusive or stalking behaviors, and unauthorized access to computers can be considered a crime in various jurisdictions, and these sorts of shenanigans usually and unfortunately only escalate.

If you do find yourself being tracked via Find My, I’d change the device passcode and Apple Account password too, and other important passwords.

Beyond Safety Check, if somebody can pick up your open phone, also enable Face ID if not already, enable attention awareness, and shorten auto-lock:

Turn Attention Aware features on or off on your iPhone or iPad Pro - Apple Support

Settings > Display & Brightness > Auto-Lock > maybe reduce to 30 seconds?

My hacked phone showed up twice on my Verizon bill, one above the other. Same phone, two times. That explained all the issues I had with my phone. My son-in-law’s face turned green when I showed him. The next day the second phone was gone, and so was the documentation I kept on my iPhone.