There is no way for you to be sure unless you plug in your phone to a PC or a device that will counter check all parts serial number in the phone, even that you cant be sure because you can copy and paste the original serial number to a new part with a serial number copy/burn device ( like screen/battery ). You can always buy a used motherboard if you stole someone's iphone and they locked the device via iTunes, but its quite a lot of work.

No point worrying, if the screen quality and color looks like the original with working true tone, Touch/Face ID work, not telco locked device, and most importantly price is reasonable with the actual phone, you are good to go. Stick to MY set.

And most of the supplier in China did an awesome job in refurbishing the device, with the most expensive one is refurbished from original used part.

Means the Face ID have been replaced as only Apple can reprogram those. Up until today still havent exist a rig/device for 3rd party people to reprogram this.

Not sure about the older iphone but on the 11 running iOS13, if swapping the battery and display without reprogramming the original parts serial number, you will get a persistent message in your setting telling you Apple is not able to verify display and battery to be original, you will lose true tone and not able to check battery health.