When you buy a used iPhone, you have no idea, if it has original screen. Its quiet easy to check.
Tap General > About. Here, you'll be able to see PARTS AND SERVICE HISTORY under your iPhone's general information. Under this, you'll see Display. If your screen was replaced using parts verified by Apple, “Genuine Apple Part” will show up next to it. 
If its a non genuine screen or genuine from another phone, it will show as " unknown part". 
Anothe way is to turn the brightness all the way up and compare it with another iphone of the same model with full brightness. you will notice original screen much more brighter than the copy screen. (Only valid for iPhone 11 Pro, 12, 13 & 14). 
iPhone 11 Pro LCD screen has 900 nits of brightness and iPhone 14 Pro Max screed has about 2000 nits of brightness.
Settings > Display & Brightness