How do I check which apps are using wifi?
Go to: Settings > Data usage – WIRELESS &amp; NETWORKS


Tap the right-top menu. It will open up a popup menu.
Check *Show Wi-Fi usage.
Go to Data Usage. You'll realize a new tab called WIFI is added nexto to MOBILE.
There, you can see all apps that use WIFI and how much data they use.



    
  Visual how-to (source: TeckFront; click images for larger variants)