Wondering just how much data you're pulling down in a given month on your iPhone? There are a few ways to check your usage, one being to head to AT&T's wireless website, log into your account, and create a billing report (under "Bills and Payments," click "Create Billing Report," and then "Data Usage Trend"), which gives you a detailed graph on how much data you're using. You can also estimate how much you use with your provider's online data calculator. Find your usage for AT&T, Verizon, or by signing in to your Sprint account.
However, this third way only requires you and your iPhone to see how much data you're devouring.
First, head to your iPhone's settings app. Tap on "Cellular," then scroll down to "Data Usage." You'll see your data usage (sending and receiving) over the cellular network for the current period, as well as call time. If you've never reset your stats, you'll find that this number will be fairly high.
Check back next week to estimate just how much bandwidth you're eating up each month, and determine if you want to switch to a less expensive plan.