Navigate to the phone's "System Settings" or "Settings." This may also be called "Options" or "Preferences." Locate the tab that will display information about phone's "Memory" or "Storage." Most smartphones will have such a feature. The number of MB used on the phone will be displayed.
Some phones come with a memory card. This card stores all the non-essential media for the phone; these are the files not required to run the phone. The card usually contains media created or downloaded by the user. Eject the card from the phone and insert the card into a memory card reader. The memory card reader will indicate how many MB are used on the phone.
Plug the phone into a computer. Many smartphones connect directly to the computer for uploading and downloading data, usually via a USB cable. When the phone is connected, it can be viewed in the "My Computer" screen as an additional device. A simple click on the device will display the amount of storage on the device and how many MB are used in the "Details" pane.
Lastly, the amount of used MB on a phone can be counted manually. Each file on the phone has a size, measured in kilobytes (KB) or (usually) megabytes. Go through your phone's files by plugging it into a computer and opening its folders as in the previous section. Total up the sizes for each file on the phone. This method is useful for helping to determine exactly which files are taking up the most space.