To take a screen shot on an android phone with 4.0 or above.
Standard methods:
- press the power button and the volume down button together for 1-2 seconds
- or press the power button for 1-2 seconds and select the screen shot option from the menu.
The power/volume down works on my Nexus 7.
However, on my Samsung 3 mini these do not work. What does work is to press the power and home buttons at the same time and hold for a second until you hear the click of a shutter. The screen shot is saved to the clipboard and into the gallery.
Unfortunately with versions older than android 4.0 you will need an app.