How can I check my data usage from my phone?

Who is your service provider?

AT&T has a free iPhone app that provides for this along with the ability to check and pay the monthly bill.

In addition, at Settings > iPhone > AT&T Services, there are options to select to get current usage.

Not sure what is avaialble with other carriers for the iPhone.

Dial *3282# and call it. It should send you a text message telling you your text message usage along with date usage. This is a simpler way.

There is also:

Settings > General > Usage > Cellular Usage

Malcolm, what version are you using? I have apdated my I-phon 5 to IOS7 and Cellular data is not there.

It is available on my iPhone running iOS 7 at Settings > Cellular > Celluar Data Usage - below the Call Time section.

Thanks Allan. I have another question. How do you close the previously opened aps?

Double tap home button and just swipe up on the application you want to close!!

Double "click" home button, you will see apps running, just swipe them up to close.

I just have figured it out. Thanks.

Dirty and Grayhair thanks.

All apps there are not running. That also now includes if you launch Settings and Settings isn't running if not removed from the multi-tasking access.

The Cellular Data Usage now shows only 2 information, Current Period and Current Period Roaming. They do not show the total usuage anymore. Anywhere i can see those info? Don't want to be over using my data plan.

That is the data usage for the current billing period with your carrier that you must reset at the start of each billing period. The total for the current billing period is what you need and what is available.

That is not 100% accurate anyway. Better to check with your carrier for what they are reporting as your data usage to date in your current billing period. My carrier includes a free app that provides this info for the current billing period along with how many days remain in the current billing period.