From checking and sending email to browsing the web to using all kinds of apps, smartphone owners use a ton of wireless data. Most of us never think about how much we use, except perhaps once a month: when our cell phone bill arrives. At that time, we learn the occasional consequences of our data use: extra charges on our bills.

Every iPhone data plan includes some kind of limit on the amount of data that can be used each month.

For some phone companies, if you exceed that limit your data speeds are substantially slowed down. For others, it means you get charged extra. But you can try to avoid these fates by checking your data use throughout the month. Each iPhone carrier in the U.S. (with one exception) gives you multiple ways to find out how much data you're using. Here's the full rundown.

AT&T
There are three ways to check your AT&T data use:

1. Your AT&T account online
2. The AT&T app, which includes data, voice, and text usage (Download at iTunes)
3. Call *data# and a text message with your data use will be sent to you

Data Limit: Varies depending on your monthly plan. Data plan options are 300MB, 3GB, and 5GB a month
Overage: $10/GB

Cricket Wireless
There are two ways to check your Cricket Wireless data use:

1. Your Cricket account online
2. Call *pay

Data Limit: Unlimited, but with throttling after 2.5GB
Overage: Data download speed is throttled to 56-100 kbps for the rest of your billing cycle, or you can purchase an additional 1GB of full-speed data for $10

Sprint
There are four ways to check your Sprint data use:

1. Your Sprint online account
2. The Sprint app, which includes all usage details (Download at iTunes)
3. Call *4 and then press 4 at the menu
4. Text usage to 1311

Data Limit: Unlimited (and though Sprint doesn't state it specifically, they seem not to throttle speeds after given usage amounts)
Overage: n/a

Straight Talk
There's very little information online about Straight Talk's policies and practices around checking your data use. It seems that they may not offer a way to discover how much data you've used. If you're a Straight Talk user and have had experience in this area, please email me to share your knowledge.

Data Limit: There are a number of reports that seems to indicate data cap at 2GB, but again, this isn't clearly explained by the company.
Overage: After 2GB, some users report having their data cut off, while others say they have received aggressive or harassing phone calls from Straight Talk.

T-Mobile
There are two ways to check your T-Mobile data use:

1. Your T-Mobile account online
2. Call #932#

Data Limit: Unlimited, though customers who exceed their data plans will have their speeds reduced (doesn't apply to unlimited plans)
Overage: n/a

Verizon
There are three ways to check your Verizon data use:

1. Your Verizon account online
2. The Verizon app, which includes minutes, data, and text messages used (Download at iTunes)
3. Call #data and you get a text with usage details

Data Limit: Depends on your rate plan. Available data amounts are 1GB, 2GB, 4GB, 5GB, 6GB, 8GB, and 10 GB
Overage: $10/GB

Virgin Wireless
There is one way to check your Virgin data use:

1. Your Virgin online account

Data Limit: 2.5 GB/month
Overage: If you exceed your monthly data limit, your download speed will be reduced to 256 kbps or less until the next billing period

Luckily, most carriers will give you some kind of warning as you start to approach your data limit. For instance, AT&T sends a text message when you've used 65% and 90% of your monthly data, as well as after you've gone over. With that knowledge, you can change your data use to avoid throttling or extra costs.