You can view and set data limits on the handset by accessing the data usage menu by accessing Settings > Data Usage. In this menu you can set data warnings, check your usage, turn mobile data off and set a data limit. The first picture below is an example of how this screen might look, it varies from manufacturer to manufacturer and depending on the version of Android. The options to set data limit, usage cycle and turn data on/off are pretty self explainatory.
Underneath the alerts section is a graph and this can be a very useful tool. You can use the slider highlighted to view usage within a particular time range. This can help you to work out where data has originated from because just under the graph is a list of apps and their respective mobile usage. The highest hitters will be listed first so it may be immediately apparent as to the cause of higher than expected usage.
Finally, you may also want to review app specific data or prevent an application from using data in the background. App specific data is listed next to the pie graph show and, again, you have the option of using sliders to select a specific date range. If an app has high background data, and you don't need it to run in the background, then check the box at the bottom to restrict this usage.