In the past, it took several steps to log on to Comcast and navigate to a check of your monthly data usage. A long, long time ago, in 2011, I wrote an article about how to log on to Comcast and "Check Your Comcast Data Usage." Forget that. Since then, Comcast has made it much simpler.
First, you will of course need to have a Comcast account with a username and password. Then, go to this URL. You'll see a page that looks like this.
After you log in, the URL above will take you directly to the "My Current Data Usage Page." My (surprisingly modest) page looks like this:
It's that easy.
Note the comment in the paragraph at the top: "...enforcement of the 250 GB data consumption threshold is currently suspended." There's more detail on the why of that in: "How Comcast is Now Handling Internet Data Usage and Caps."