Take a look the documentation for InputEventMouseButton.
You’re looking for the button_index property. You can check it like this:
if event.type == InputEvent.MOUSE_BUTTON:
if event.button_index == BUTTON_LEFT and event.pressed:
# do something
Of course you can also use InputMap actions for this which is often easier, especially if you want to support multiple input methods (mouse, keyboard, gamepads) for the same gameplay mechanic.