How do I know if my iPhone screen needs replacing?
Green or coloured vertical lines, spreading black patches, dead strips that ignore touch, ghost touches that happen on their own, or a display that lights up but shows no picture all point to the front assembly. A phone that is completely dead, or that shows a picture but will not charge, usually points somewhere else.
Symptoms that mean the display itself
Vertical green or white lines after a drop are a classic failed OLED connection. Spreading dark blotches are damaged OLED pixels. A bright but blank screen is normally the display's own driver. Ghost touch — the phone opening apps or typing by itself — is the digitiser layer, and it is part of the same assembly.
Dead strips along one edge, and touch that works everywhere except one band, are also digitiser faults. All of these are solved by replacing the front assembly at the published price for your model.
Symptoms that point elsewhere
No power at all, no vibration and no charging light is more likely to be the battery or the board. A screen that works but the phone restarts under load is usually the battery. No sound, no charging or no signal is not a display fault at all.
If your phone has been wet, treat every symptom as suspect until the board has been cleaned — fitting a screen onto a corroding board hides the real problem.
When to get it inspected instead of guessing
If the symptoms do not clearly match a display fault, an inspection is the cheaper route. That is what the diagnostic service is for: identify the actual fault, then quote the actual repair, rather than buying a part on a hunch.