Upon taking a screenshot the following message will be displayed "Saving screenshot ...". You can view your screenshots by going to "Gallery" > "Screenshots"..
Method 1- (All Android devices)
Simultaneously press on the Home and Power button. You'll hear the camera shutter sound that indicates that the screenshot has been saved.
Method 2 - (Samsung Galaxy S5)
Go to Settings > Motions and gestures > Capture screen. By enabling the Capture screen feature, you can take screenshots by swiping the side of your hand over the screen (from left to right or vice versa). The screenshots are automatically saved in Gallery > Album > Screenshots
Method 3 - (Samsung Note)
Press the button of the S-Pen stylus. A small circular menu (Air Command) will be displayed on your screen. Tap on "Screen write" to take a screenshot of your current screen.
For Nexus 7 and 9 simultaneously On/Off and volume button.