An add-in could find out this information.
To find out which pools an activity is in: You would need to find all the diagram objects that represent your activity element, find out which diagram they are on, find all the diagram objects that represent pools on that diagram, check that the activity's diagram object's left, right, top and bottom values are all within the pool's diagram object's left, right, top and bottom values.
To find out which activities are in a pool is a similar process: Find all the diagram objects that represent your pool, find out which diagram they are on, find all the diagram objects that represent activities on that diagram, check that the activity's diagram object's left, right, top and bottom values are all within the pool's diagram object's left, right, top and bottom values.
It could probably be done with a single custom SQL query (as Geert suggested) but it would be a complex one. Fields to look in include t_diagramobjects.RectTop, .RectLeft, .RectRight, and .RectBottom (be warned that vertical co-ordinates are negative numbers)