How to tell the difference between fat necrosis and normal fat deposits?

Fat nec. vs deposit: Interesting question? Fat Deposits are accumulations of fatty tissue either loosely or in a collection, i.e. lipoma. Vs Fat Necrosis is a disruption of the architecture of fat itself by the death of the fat or adipose cell. Thus leading to indentations or depressions in the area of necrosis. Better explanation with an in person visit to your Plastic Surgeon!

What part: of the body? If it is in the breast the diagnosis can often be made with mammography, and almost always with breast MRI.