I want to convince someone that this image belongs to the 42mm Apple Watch.
However I couldn't distinguish between them yet.
How to prove this is a 42mm or 38mm Apple Watch
apple-watch
Related Solutions
The health data is automatically added by the watch. If you can see heart rate data, this means your Apple Watch is sending data to the Health app properly.
The message on the screenshot you posted is a bit misleading. In this answer you'll learn how to verify that the heart rate data is being transmitted correctly and how to shut off heart rate data collection.
Seeing Heart Rate data
Open the Health app on the iPhone. Select the Health Data tab. Navigate to Vitals > Heart Rate:
If you see data, that means your watch is properly sending data to the Health app. You can make this visible on the initial screen for the Health app by toggling the Show on Dashboard switch.
Disabling Heart Rate data
You can turn off Apple Watch health data collection. Open the Apple Watch app for iPhone. Select the My Watch tab. Navigate to Privacy > Motion & Fitness. There, you can turn off Heart Rate or Fitness Tracking:
I got the same message on my Apple Watch and responded "Don't Trust" since I had no idea what computer was trying to connect. Then I read this article so opened the Console app on my Mac. Initially it didn't show my Apple Watch under Devices. I think I had to connect my iPhone to my computer first before both my iPhone and Apple Watch showed up, the latter with an exclamation icon due to the "Don't Trust" response.
I disconnected my iPhone from my computer and restarted the iPhone. I then reconnected it to my computer to check the status in Console again. I got the "Trust this computer" prompt on my Apple Watch again. After responding "Trust" this time and entering my PIN, the warning icon disappeared in Console after a few moments.
It should be noted that there may be delays before the Apple Watch shows up in Console. I think opening Xcode seems to hasten the discovery process but I'm not sure. Also, I have yet to see any Apple Watch log entries in Console.
Best Answer
While that is just a rendering of the Apple Watch, not an image of an actual Apple Watch, the band is a Modern Buckle, which is only available for 38mm Apple Watches. So the Watch in the rendering is definitely a 38mm.