Ok. To do it, you need to use grep terminal application on your phone (the easiest way with rooted phone with busybox package). Let do the same steps as below:
C:\Users\Hasan>adb shell input keyevent KEYCODE_DPAD_LEFT
C:\Users\Hasan>adb shell
shell@android:/ $ su
su
root@android:/ # dumpsys window windows | grep -E 'mCurrentFocus|mFocusedApp'
dumpsys window windows | grep -E 'mCurrentFocus|mFocusedApp'
mCurrentFocus=Window{42079c60 com.jrummy.liberty.toolboxpro/com.jrummy.apps.rom.toolbox.RomToolboxActivity paused=false}
mFocusedApp=AppWindowToken{4283d918 token=Token{426709a8 ActivityRecord{41e37c60 com.jrummy.liberty.toolboxpro/com.jrummy.apps.rom.toolbox.RomToolboxActivity}}}
root@android:/ #
Now you'll see which activity has focus :)