How can I verify that my iPhone is legitimate?

I found the following articles with some good information:

How to notice the fake and original iPhone - Apple Community

https://convergeddevices.net/how-to-spot-a-fake-iphone/

If you determine that you have a counterfeit product, you should report it promptly at:

Apple - Legal - Counterfeit Questions

A guaranteed check is to launch the App Store app. If it goes to the actual App Store, it's real. If it goes to the Google Play store, or throws out a fake message that it can't connect, then it's a fake.

Such a fake message would be an impossibility. You can't have internet access and not be able to access the App Store at the same time.

I plugged the serial number into Apple's serial number checker and that site accepts the serial number as being that of an iPhone 15 but reports the Purchased Date as January 01, 2007.

The phone is legitimate, but it has been refurbished / remanufactured either by Apple, an Apple approved company, or a company not approved by Apple.

If you can provide the Model Number of your iPhone.......Settings > General > About > Model Number.......that might tell us a bit more about who refurbished the phone. We do NOT need the serial number, so please do NOT post that for security reasons.

The original warranty on the phone has expired, but the seller that you bought from might have offered a warranty on the phone. Check with them on that.

That model number (5TM43LL/A) would indicate that it was refurbished by an Apple approved facility. A warranty, if any, would be provided by the retailer.

Then it's a genuine iPhone. Android phones cannot connect to the App Store.