If there is a system set delay it will show you clearly on the home screen that that is the case.
If you have smart watering enabled for the system it will set automatic rain delays for you.
If you have smart watering disabled for the system and you have rain delay notifications turned on it will send you notifications suggesting you set a rain delay using the button in the app you mentioned appears on the home screen of the app just above the navigation bar.
If you have a physical rain sensor installed, the system will only show you that there is a sensor delay when a scheduled program (smart or custom) attempts to run, it will then show on the timer as SENSE (if I remember correctly) and in the app it will show you a message that at rain delay has been triggered by a sensor.
If you want to run smart watering and use a physical rain sensor and don’t want the system to set delays for you then you can go to Settings > Devices > Select your timer then go to Weather Adjustments and adjust the thresholds to a level that it would have to be a rain of biblical proportions before the system would set a delay for you.
Hope this helps!