What is the test for determining whether a monopoly is a public utility?

In Munn v. Illinois, 94 U.S. 113 (1877) [Munn], the United States Supreme Court held that the grain elevator industry operated as a “virtual monopoly” and was therefore impressed with public utility duties. The grain elevators in question had market power over an essential link in the transportation chain and, on that basis, the court applied public utility constraints on the industry. At 131-132, the court said the following: Thus it is apparent that all the elevating facilities through which these vast productions “of seven or eight great States of the West” must pass on the way “to four or five of the States on the seashore” may be a “virtual” monopoly. Under such circumstances it is difficult to see why, if the common carrier, or the miller, or the ferryman, or the innkeeper, or the wharfinger, or the baker, or the cartman, or the hackney-coachman, pursues a public employment and exercises “a sort of public office,” these plaintiffs in error do not. They stand…in the very “gateway of commerce,” and take toll from all who pass. Their business most certainly “tends to a common charge, and is become a thing of public interest and use.”