How to tell if bath waste connected to soil stack

[WHoops]

In first floor flat (built 1980's) the bath waste pipe goes through the wall very close to where the soil stack is. It's all behind boxed in work.

Is there any way to tell if the bath waste is connected to the stack or the pipe goes out to a gully without having to rip up a lot of boxing in?

For example; if I flushed the loo would you expect to see the bath waste water in the trap ripple a little from slight suction created if it was connected to the soil?

Thanks

 

[SimonG]

What's the problem?

 

[WHoops]

Deciding which traps should be fitted on the bath; bath clearance is low

 

[SimonG]

I know people wont like this, but fit whatever fits in the space you have.