The means to establish rough consensus was described by the IETF (1998) as follows:
Working groups make decisions through a "rough consensus" process. IETF consensus does not require that all participants agree although this is, of course, preferred. In general, the dominant view of the working group shall prevail. (However, it must be noted that "dominance" is not to be determined on the basis of volume or persistence, but rather a more general sense of agreement). Consensus can be determined by a show of hands, humming, or any other means on which the WG agrees (by rough consensus, of course). Note that 51% of the working group does not qualify as "rough consensus" and 99% is better than rough. It is up to the Chair to determine if rough consensus has been reached (IETF Working Group Guidelines and Procedures).
The phrase is often extended into the saying "rough consensus and running code" , to make it clear that the IETF is interested in practical, working systems that can be quickly implemented. There is some debate as to whether running code leads to rough consensus or vice versa.