Sprint will show you your current data usage as part of your online monthly bill. You can also send a text message to some of the U.S. carriers using a special number and they will (almost) immediately send you back your minutes used, data used, or some other statistic relating to your account. T-Mobile is great with this texting feature, and I do believe Sprint offers it as well. Never used it myself with Sprint (my current carrier) as I have the unlimited plan.
