About a waste drain

I am wanting to make a downstairs bathroom ...at the side of my house there is the soil waste pipe for the upstairs toilet but where i want put my downstairs toilet is to far away to connect to yhe upstairs soil pipe ...but where i want put my dowstairs toilet there is a drain ouside right next to the wall where im go be putting my downstsirs toilet ...it has a drain going into it from the gutters of the roof water waste ...how would i no if that drain is connected upto the waste pipe what the soil pipe go into from the upstairs bathroom so i can use that drain to connect my waste into from my toilet what i want to put downstairs so i no it is connected to the pipeing under ground that the other upstairs soil pipe is connected to