How do I know if a soil pipe is leaking outside?

It should be easy enough to determine where the leak is coming from. Try putting some food coloring in the toilet then flush it, it should make it easier to see where the leak is coming from. Then if it appears to be an easy joint or section, replace it.

Leaking Soil Pipe

There may be a slight leak further up the pipe where the toilet, bath, and sink run into it. get a friend to Run the bath only while you check the waste pipe for the bath where it runs into the soil pipe. Then do the same with the sink, and finally the toilet. The leak can be on the joints for the waste pipes and running down the soil pipe. However if you are sure it is leaking at the joint you mentioned, then buy a roll of "Denzo" tape from a plumbing merchant and wrap it around the leaking joint. Toilet water is 95% clean water when you flush the toilet.

Well its interesting that you have pvc pipe going to your bathtub. Most houses are copper pipe. I would recommend starting over. Cut out your old fittings you put on and also make sure your using pvc glue or pvc cement glue. Also buy the purple primer which you use first. PVC glue takes a good amount of time to set and take pressure so read the instruction on the glue for how long you need to let it dry. You may have turned the water on too soon. Put primer all the way around the pipe and inside the fitting. Put glue all around the pipe but I wouldnt worry about any in the fitting. Make sure you have glue all around the pipe for a good seal.

go out on a dry day when all the damp is dry get someone else to run bath and sink water and flush the toilet if you see any drips or damp reappear you have a leak if not you don't

Call a plumber. You say a joint is leaking, so the plumber can fix that and also tell you if the rest of the pipe is OK.

You need to elaborate your question but still, you can use a tissue paper to detect the leak. So, take a tissue paper and work your way to the pipe you think is leak. When the tissue paper is is moistened, you will know where the leak really is.

The soil pipe IS leaking, get a plumber to make good the joint. It is dangerous to allow sewage to seep from a defective joint, ( e-coli etc )

Soil Stack Leaking

dye ..just put some dye in the toilet bowl ..flush and there's your answer ..however the toilet connection would have to be below the joint not to leak ...joints are not fussy they leak all types !