The model number of an Apple refurbished watch should start with 'F'. If the watch was repaired or refurbished by Apple.
You can check out the body including the screen for any damages or cracks so you will get clarity about the hardware.
Ask for an original copy of the bill and box to confirm whether the watch is genuine or not. You will also get the details of the warranty period.
You can verify the date of purchase by entering the serial number into a website. You will get the required information about the refurbished watch.
As per the policy, all refurbished Apple Certified Refurbished products come with a one-year limited warranty.
The packaging should be the same as; the retail packaging. You must check a small sticker that says "Apple Certified Refurbished".