New drainage is always split so that your rainwater and surface water go down one set of pipes, and your fowl (sewage water) goes down a different set of pipes. This is set out in the building regulations.
Sometimes, in an older property the drainage can be combined. This can also happen by accident if an extension is built (especially if it was prior to stricter building control regulations). It is important to ensure no rainwater enters your sewage treatment plant or septic tank, therefore testing to see where your water goes is a good idea if you are replacing an old sewage system
In this article we explain a method to test if rainwater is entering the fowl drainage system, this is best done when it is not raining).
Sometimes it is not possible to spray water onto a roof. Therefore try and find the nearest manhole to the down pipe. Creating a drawing can really help to try and establish the route of the underground pipes. Drain tracing dye is also very useful to determine the source of water. Always keep in mind that it can take longer than you think for water to flow down pipes, and it is useful to have 2 people to do this job.