I agree with Bimr, smoke test is much better and efficient way to know about the major problems in the sewage. As the flows of the sewage vary with time and with different weathers.
John
http://www.minidiggerhire.ie/#/skip-hire-dublin/4547725214
These days most of the manholes are completely sealed packed. We have generally circular manhole covers. And about the holes, the presence of holes make the air pass through them so that the pressure created by the gases generated from the sewage do not exceed.
Thanks Bimr for sharing a great...