Q: How can I know my iphone 6s whether is fake or original?

Find the serial number or IMEI on your iPhone, iPad or iPod touch - Apple Support

https://checkcoverage.apple.com/

Original == refurbished. You have no reliable or trustworthy way of distinguishing between the two and Apple makes no differentiation if under warranty or AppleCare+ coverage.

First consideration is where did you get it?  Did you buy it from Apple or an Apple Authorized reseller?  If so, it won't be fake.  And unless it was a replacement under warranty or out of warranty, it won't be refurbished as Apple does not sell refurbished units at retail.  A reseller may sell a device as refurbished or reconditioned or pre-owned, but that would have been done by them or their contractor, not Apple So you'd need to ask them about that.