They tell you...
at 65%, 90% and 100%. I'm not saying it's exactly right not to give you a meter, but at least they tell you.
What baffles me is that since they are obviously keeping track, they can't they make a simple few lines of code to output that into a number that you can see on a web page?
While the concept of metering is wrong, especially given that U-Verse isn't a shared medium like cable, their rates are very reasonable compared to the competition, and $10/50GB isn't going to cause anyone to have a massive phone bill. I'd rather they work on global roaming charges on the cell phones than worry about this.