If you have a limited data plan or a data cap for your iPhone, it’s a good idea to keep an eye on where your cellular data is going. But how do you find out which apps use the most data? How much data does Spotify use? What about YouTube? If you can find out which apps are sucking your data, you can turn off Cellular Data usage for them, which means you’ll only be able to access the apps over Wi-Fi. If you have unlimited data, how to check and manage data usage on an iPhone isn't a big deal. But if you’re trying to stay within a certain amount of gigabytes each month, this data saving tip will make it easy. Here’s how to check what apps are using the most data on your iPhone.