I live with data-limited WiFi (I live on a mountain, it's ok I knew what I was sacrificing). I get 60gb a month for WiFi and 15gb for my phone.
What that means is that I am a data hawk. I have about a dozen devices which use data and I can check each one individually to make sure nothing will cause an overage. Here is what I know:
1) The Apple Watch does connect to the internet independent of the iPhone while on WiFi, but only uses about 350kb (that's kilobyte, tiny) a month.
2) So, knowing that, I can safely say 99% of the Apple Watch's data usage (WiFi or LTE) goes through the phone.
3) During periods when I don't use my phone for anything, my data usage is very small. Basically, polling for email/iMessages/snapchats etc. Usage during these periods is around 2-3mb per hour or less. This number did not seem to change when I added the Apple Watch three months ago.
4) Rationally, I don't see the Watch adding anything to data usage beyond polling for weather data more frequently. But weather data is very small, probably less than 100kb per check (it's so small I have very little ability to estimate it, and my estimate is based on my smart thermostat's weather checking, not my Apple Watch).
If you went over your data plan by 12gb, I'd say it's a safe bet your watch had nothing to do with it. If you want to be a data hawk with your phone (since you probably have unlimited WiFi you lucky person), try using the battery usage breakdown in Settings. If a rogue app is running through data, it's more than likely eating your battery too. And watch out for the spinning circle next to the LTE/WiFi icon, especially when you don't think you're drawing data. Also be wary of anything which automatically downloads (turn LTE data off for all that stuff), and anything with video applications. You can't go 12gb over without video playing a role somewhere in the equation.