If you buy from Apple's Certified Refurbished Store, you get the same terms as if you were buying a new product from Apple. You get the 14 day return policy, 1 year Limited Manufacturers Warranty, and the option to add AppleCare+ to it. All devices also come with a new battery and other parts. In short it's basically a new Watch. If anything Apple would be the most reliable to buy from as other retailers typically don't refurbish like Apple does, and for third-party there's no saying in whether the Watch has certain defects or not.
As for seeing if this feature will work correctly before purchase, there really is no way to test it. The certainty comes from buying from a reputable source. Apple wouldn't offer a new product in the box's policy to refurbished if they didn't think it was like new.
Start here: Refurbished Apple Watch Deals - Apple