Question: how to check mobile data usage on galaxy s6?
Answer: From a Home screen, navigate: Apps > Settings. > Data usage. ... 
Tap Mobile data usage. To see Wi-Fi data usage, tap Wi-Fi data usage in the Wi-Fi section.
Tap an app (below the usage graph) then view the usage. To restrict the app from using data in the background, tap the Allow background data usage switch to turn off.