Any apps you use that need to pull information from online will use your mobile data unless you're connected to a Wi-Fi network. The easiest way to make sure you don't go over would be to simply disable your mobile data altogether aside from the times that you absolutely need it. You can do this on your phone by going to Settings, Cellular, and then disabling Cellular Data. For that matter, that Cellular screen should also have a list of apps, with numbers beneath them to let you know how much mobile data each one has been using. That may be helpful in letting you know what normally uses your mobile data.