If your iPhone is having issues, it might need replacement parts. Here are a few ways to tell:
1. The issue is not resolved by a restart or reset.
2. You see physical damage like cracks or missing pieces.
3. Your iPhone isn't responding to touch or button presses, or it's responding inconsistently.
4. The display is blank or has strange colors, lines, or patterns, or the backlight isn't working properly.
5. There are performance issues, like lagging, freezing, or battery drain that can't be explained by normal useage patterns.