Chosen Solution
I purchased a used iphone X with cracked glass and no Face ID. I planned to replace with an OEM screen and could have done w/o the Face ID. But now I realize that even the true tone is missing. So is my display already a replaced one or is original? How can I make out this?
If face Id and true tone are both missing the screen has definitely been changed. Unfortunately face Id is not repairable unless you have the original parts. It sounds like they tore the cable to the proximity/floor sensor and earpiece speaker. It possibly to repair that if you can find someone with the skills. The front camera and those sensors are paired to the motherboard and if one is damaged or changed then you are out of luck. Unless you want to trade it for a refurbished one from apple and pay a few hundred dollars. But if you can find a repair shop that has the correct equipment you can get true tone back.
