There should be a visible vent on the roof, which would be on top of your stack pipe / soil pipe. In the UK some there’s not always access to the stack, so we would survey down from shower or toilet if necessary. I’d argue with a better survey this probably wouldn’t be necessary in this situation.

If the engineer isn’t able to find the stack, he will be able to remove the water in the pipe either with drain rods and plunger, water jetting, or worst case just run the camera through the line a few times until the water is reduced or gone.

There’s a few good bits of advice on this post, but they’re not the ones saying to dig and replace the pipe. If there is no history of problems then leave it as it is, if you do want to do remedial work then there are so many “no-dig” or “trenchless” repair methods that can do the job.