Question for cellphone repair shop owners about warranty?

Hi everybody,

I would like to ask question related to warranty

Would like to hear back from repair shop owners with experience in this business,

I came across situation where customer came back with issue on LCD after screen replacement was done on his/her iPhone 6 about month ago,

My question would be - how do you people know that the problem is factory related fault other than physical damage if the front glass isn't cracked or chipped but vertical lines appeared after a month use???

Would you get it fixed under warranty?

What's the warranty policy you are fallowing in your repair shop?

Thank you in advance,

Regards

Janeks