As far as I know, Windows Phone Doesn't notify you when it uses data. How can I tell if Whatsapp downloads messages in background or only when app is opened? If it is doing in background than how it is achieving it P.S i am using Wp8.1 Dev preview with all latest updates
Usually, WhatsApp downloads messages in background. You can notice that the app gives notifications for messages even when you haven't opened the app.
In cases you have activated battery saver, WhatsApp doesn't run in the background, unless you have configured it to give notifications (this rarely happens). You can notice that there is no notifications arriving until you open the app.
This is supposed to be how it works. But, I frequently encounter a problem with the app when, even with battery-saver mode off, I don't get messages until I open the app. Most of the times WhatsApp notified that an error has occurred, and that I need to restart the phone to get notifications. I don't know whether it is a bug with the app or the OS.
I have also noticed (many of us might have noticed too) another problem with notifications. It doesn't show all the notifications. I mean it skips a handful of notifications about messages.
Actually, the app can behave of the both ways: If you use WP8.1, go to battery saver app and see if what'sapp is allowed to run in background. If yes, well, the app is downloading messages in background. A good way to notice that is paying attention to your notifications (do you receive what'sapp notification even if the app are not in the foreground?) If the app are not allowed to run in background, when you open it, it will download all the messages send to you since the last time you opened it.
Skipping the obvious stuff, you can check the official WhatsApp FAQ page with more informations regarding WhatsApp for Windows Phone.
Also, there are some reports that WhatsApp can misbehave if Wifi and Mobile data network are activated at the same time