My 17 inch laptop shows only colored lines on it and when I took it in for warranty the guy says it was impacted by something and probably wont be covered. Iam almost certain it was not damaged but can technicians tell if it was damaged or not.
If there is apparent physical damage to the screen area, they know it was damaged. If there is no evidence of it being dropped or hit then there is no way for them to tell.
Is there? A dark spot indicates pressure damage, and while that may not even have led up to the damage that caused the lines it might very well void the warranty.