Canon puts a punch mark on the rear of their lenses, I suspect they either put a paint dot in the battery compartment or a punch on the serial number plate.
Being refurbished merely means its a used camera that has been inspected and tested. If you are buying a used camera, there is no warranty, so refurbished really is meaningless.
I can certainly live with a refurbished. I just want to make sure the seller is being honest and I get what I pay for.