For Mac/Linux debug keystore, the Android docs have:
keytool -exportcert -list -v \
-alias androiddebugkey -keystore ~/.android/debug.keystore
But there is something that may not be obvious: If you put the backslash, make sure to do a shift + return in terminal after the backslash so that the second that starts with -alias is on a new line. Simply pasting as-is will not work.
Your terminal (if successful) will look something like this:
$ keytool -exportcert -list -v \
? -alias androiddebugkey -keystore ~/.android/debug.keystore
Enter keystore password:
The default debug password is: android
Side note: In Android Studio you can manage signing in:
File > Project Structure > Modules - (Your App) > Signing