Pit bull is the common name for a type of dog that includes the Staffordshire Bull Terrier, American Staffordshire Terrier and American Pit Bull Terrier. The American Bulldog and Bull Terrier are sometimes included.

A pit bull type dog can be identified by a cluster of attributes by animal experts like veterinarians, animal control, and shelter workers. The major characteristics are a blocky or wedge-shaped head, small wide-set eyes, high folded ears, muscular body, short coat, strong neck, broad chest and well-muscled hind quarters. While other types of dogs have one or more of these characteristics, a pit bull type dog will have most of them.

Read more about pit bulls and their history
http://www.daxtonsfriends.com/american-pit-bull-terrier/

Pit bulls and the names they go by
http://sruv-pitbulls.blogspot.ca/2015/04/by-any-other-name.html

Identifying pit bulls
Many pit bull advocacy groups argue that it is impossible to identify pit bull type dogs. At the same time, all pit bull owners believe they are able to identify a pit bull type dog. In addition, there are more than 12,000 shelters and adoption agencies for dogs in Canada and the United States with tens of thousands of staff members, and they are all able to identify pit bull type dogs.

The ASPCA has stated pit bulls can be identified correctly 96 percent of the time.
http://www.aspcapro.org/blog/2013/09/25/bully-this—-results-are-in…

The courts state clearly that dog owners of “ordinary intelligence” can identify pit bulls.
https://scholar.google.com/scholar_case?case=1496137140642653899&q=State+of+Ohio+v.+Anderson&hl=en&as_sdt=2%2C44&as_vis=1

SAMPLE PICTURES OF DOGS IDENTIFIED AS PIT BULLS BY RESCUE GROUPS ON PETFINDER