There is no easy way to list all the missing ones, they are plain missing.
It is however possible to list the gaps with a query or two.
you simply make a query that does the calculation:
NextAssessionNumber: Assesion_number + 1
Then take this and join it back into your original query, this would be an unmatched query and you can build it with the wizard if you want. However it will show only the start of the gap... if you want/need all missing thats gonna take (some) more work.