It is not difficult to tell the difference between skin and fat. I think the more important point is that there is a layer of superficial fat which should not be removed with liposuction or you will develop contour abnormalities. On the other hand, the skin and this superficial layer of fat are removed together in a tummy tuck. Hope this helps.
An experienced plastic surgeon can determine the difference between fat and loose skin in about 5 seconds via physical examination. It is done by feel, and how much can be pinched, when compared to areas of the body that don't normally have as much fat. It can take much longer to convince a patient that loose skin is loose skin, but the determination is the most important detail in deciding how to get a pleasing result. We all want good results, and we all want to do the smallest procedure which will give good results. For the tummy, this boils down to liposuction vs. a tummy tuck. When skin is tight and fullness is from fat, liposuction works well. It is a quicker procedure with a faster recover. When skin is loose; however, liposuction can be a disaster. It can cause irregular folds in the skin that, while less full, looks less than attractive. The advantage of the tummy tuck is the ability to tighten, as well as remove. The abdominal muscles and the skin can both be tightened with a tummy tuck. It is a longer, more expensive procedure that has a longer recovery, but it is also doing a lot more than liposuction can. The downside that most patients worry about is the scar. A well placed scar beats loose skin, especially liposuctioned loose skin, hands down. If you have had liposuction of the abdomen and the skin is loose and irregular, you may stil be a candidate for improvement with a tummy tuck.