How to tell if used iPhone 4s has replaced glass screen?

I found a local seller with a used iPhone 4s still under warranty. Seller said it was all original equipment. Had a slight problem with the phone and took it local Apple store. Genius guy said the glass screen had been replaced with a non-Apple screen and thus the warranty is void. He showed me what to look for. When you rub your finger very lightly over the home key an original iPhone has a very smooth feel to it as your finger crosses over the home button to the body of the phone. On my phone when I slid my finger across I could feel a sharp edge between the home button and the body of the phone. My friend's iPhone 4s is brand new and the glass never replaced, but it also has a feel of the sharp edge. I was wondering is this really true about replaced glass screens or is there another way to tell if a used phone has a replaced glass screen...Thanks.