Meet at a coffee shop or a "Safe Exchange Zone" (many police stations have these).
Ask for a photo of the back of the watch. It lists the series (e.g., Series 9), case size (e.g., 45mm), and material (e.g., Aluminum vs. Stainless Steel).
Look for deep scratches on the glass (which can affect water resistance) and check the "Digital Crown" (the side dial) to ensure it rotates smoothly and clicks.
When you turn the watch on, it should immediately show a screen asking you to bring an iPhone near it to start setup. If it asks for a passcode or shows a clock face, it’s still locked to the seller.