The way that I track my data useage is with the My Account app on my mobile device. Using this approach, when you open up the App, select the three horizontal lines in the upper left hand corner of the screen to open up the menu. Select the Internet option and it will open up the screen and at the bottom show you the data useage so far on your billing cycle. Bear in mind that there may be a bit of a time lag on the useage meter but it is close enough to know if you are getting near your limit.
Under the old xfinity.comcast.net web site format you used to be able to check it there but I have been moved over to the new format and I am not having any success at locating where you can find that information on the actual web site.
FWIW, while I haven't tried it under the new web site format, I do know that there should be a way to set up alerts so that you are provided some notification in advance as you reach certain levels of nearing the data cap level in a billing cycle.
When I try the links in the previous post, I keep getting redirected to the User Settings page. Anytime I try to acccess any information about my account online, I get redirected to the User Settings page. The best I've managed (when I clear my cache or browse incognito) is get a few seconds of the new xfinity homepage before being redirected, yet again, to the old User Settings page. At this point it is no longer possible for me to check my bill or usage online, which is starting to feel like an intentional "error" on the part of Comcast so I that I can't hold them accountable for overcharging me.