Drag the top of your phone screen down until you see the 'settings' cogwheel. Click on that, then scroll until you see DEVICE CARE in the drop down menu. This will start automatically, scoring your phone's overall health (100 indicating that your device is optimised). below that, you will see MEMORY - click on that, and it will tell you how much ram is being used, and how much you can save. that should help clear the cache.