What is the test for a breach of contract?

The author then goes on to quote from Goss v. Nugent (1833) 110 E.R. 713 as follows: [A]fter the agreement has been reduced into writing, it is competent to the parties, at any time before breach of it, by a new contract not in writing, either altogether to waive, dissolve or annul the former agreements, or in any manner to add to, or subtract from, or vary or qualify the terms of it, and thus to make a new contract; which is to be proved, partly by the written agreement, and partly by the subsequent verbal terms engrafted upon what will be thus left of the written agreement.