This msg contains more naturalistic descriptions, so please read only if you are ready for it.
I call this building a relationship with my "insides". It happens overtime. One learns to know the feeling of complete evacuation of the intestines. Sort of feeling empty in these places of the abdomen. Easier to grasp is to follow the evacuation by having a look in the toilet bowl. The majority of ppl would be very careful about how their food looks like before eating it, but once it passed the stomach have no clue or do not check if everything seems to be normal on the other side. I recommend watching a past Dr oz show about stool, feces and evacuation. Generally the healthiest, foul-smell free stool is with high fiber diet, when it literally decomposes to small particles in the toilet water, since there is no glue-ie stuff to glue it, therefor the absorption of nutrient is maximal (instead of the "glue", as in form gluten, mucus etc, holding in itself nutrients) and the mop-like effect of the fiber in the colon is also impeccable. A month of this full-fiber diet, very little to none would be left in the GIT as mucus and debris.
Colonics are another way of doing it. Maybe the 3rd or 4th, spaced out by 4 days, given a good fiber rich diet is in place, would start feeling somewhat OK, the first few would be with partial results and in most cases - uncomfortable. They ar worth, though if the stagnation of GIT is important. For a colonic one needs a urinary catheter as an extension to the water irrigation kit, to replace the tinny end-piece it provides. The catheter will get past the rectum, need to insert about 25 cm or 30 cm depending on the body size, so that the urge to evacuate comes later and therefore more water can get in and past the sigmoid colon - best is feeling the water up to the appendix area.
Kombucha or magnesium citrate drinks everyday may have similar effect.
Another not so complicated thing is to have mono-diet for 3 days in a row, e.g. watermelon, or only green juices (with some fiver in them) to satiation, as many times a day you need. It is a sufficient time to get grasp on what you are evacuating, since I nearly guarantee, you are going to start seeing which is water melon (or green fiber) and what is old stuff that these clean out on their passage. Depending on how much the old stuff is and whether it looks like fecolith (very hard stool, sometimes sitting there for decades - like rests of chard meat, undigested particles etc) you may need to do these clean ups multiple times until your insides are perfectly clean. For many, eating the high fiber diet becomes further a lifestyle, something they don't force themselves but like to do. Any other variant of diet, containing either or all of: refined grains and rice, meats, dairy would inevitably generate some or a lot of debris and mucus, that sticks to the intestinal wall.
When the abdominal girth is rather big, it is usually not just fat that makes it big; it is also due to pounds of retained food remnants that some people never clean up their entire lives. The so famous westerns' actor John Wayne, Elvis and the majority of ppl seemingly, fall victims of impacted old matter in their insides. You may argue some of it can be myth or revenge from hater, but I bet it is true in many cases - do you remember the singer's abdomen in his last days? - he needed heavy belts to support the weight of all that toxic debris:
http://www.thehealthyhomeeconomist.com/elvis-died-of-constipation/
https://s-media-cache-ak0.pinimg.com/236x/1d/12/3b/1d123be55a8f8d944a1535a0490120e1.jpg
It is not yet accepted as a standard, but it is pretty much no brain-er that evacuating the colon is supposed to be as many times a day, as a person eats. Also, if the quantity evacuated is not coming close to the supposed dried-out mass of the food we eat, then constipation is there.
Additionally it may be useful to research the topics of probiotics, prebiotics, enzymes, living food, high fiber idet, whole grains.
Hope this was useful,
DL