How to identify whether an iphone has been used previously?

I recently bought an iphone 6 128gb from ebay and the seller stated that its a brand new replacement iphone from apple, meaning that he hasnt used it before and has confirmed that its not a refurbished phone, and he will provide me with the letter and original receipt from apple for all of what he is stating, the apple warranty will expire in 1 month meaning that he owned that iphone for 11 month.

My worry is that he has used that iphone all that time but kept it in good condition and now cleaned the phone and put the stickers and selling it as new, especially that he stated that he is selling the iphone because he already bought iphone 7plus and iphone 7 only came out last September.

So i'm just really wondering if there is a way to find out if this phone has been used before other than checking how the phone looks and performs, especially when i paid £380 for it.

I hope i've been clear in explaining my query.

Thank you.