Besides using Android Studio, you can also take a screenshot with adb which is faster.
adb shell screencap -p /sdcard/screen.png
adb pull /sdcard/screen.png
adb shell rm /sdcard/screen.png
Shorter one line alternative in Unix/OSX
adb shell screencap -p | perl -pe 's/\x0D\x0A/\x0A/g' > screen.png
Original blog post: Grab Android screenshot to computer via ADB