If you are using android studio use simple step
Tasks
treeandroid
-> signingReport
and see the magic Result Under Run Tab If Android Studio < 2.2
From android studio 2.2
Result will be available under Run console but use highlighted toggle button
Or
Second Way is
Create new project in android studio New -> Google Maps Activity
then open google_maps_api.xml xml file as shown in pics you will see your SHA key