A contusion can be differentiated from postmoem hypostasis by doing:

In hypostasis, on incision blood is seen blood vessels, which can be easily washed away. Subcutaneous tissues are pale. In contusion, the incision shows extravasation of blood into the surrounding tissues which is firmly clotted and cannot be washed by gentle stream of water. Subcutaneous tissues are deep reddish- black. Ref: The Essentials of Forensic Medicine and Toxicology by K S Narayan Reddy, 27th edition, Page 163.