A contusion can be differentiated from postmoem staining by doing:

While taking an incision on contusion, 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. On incision in postmoem staining, blood is seen in blood vessels, which can be easily washed away. Subcutaneous tissues are pale. Ref: The Essentials of Forensic Medicine and Toxicology by KS Narayan Reddy, 27th edition, Page 163.