How do i check my phone 5S is original or refurbished

Well the only way to get a refurbished iPhone is:

To buy one online from a career or eBay etc. which is labeled "refurbished"

To get your iPhone replaced at the Genius bar.

Otherwise your phone is not refurbished.

A while back Genius bar phones had the serial number start with 5K. I am not sure if this is still the case or true with refurbished phones from other sources.

So think to your self how did you obtain this device.