According to FindLaw.com, verbal agreements are binding. However, they are difficult to prove. As with any legal dispute, the enforceability of the verbal contract depends on the existing evidence.
Rules of evidence vary by state, but are typically less stringent in civil cases than they are in criminal cases. USLegal.com provides evidence rules broken down by state. In general, a criminal case must be proved "beyond a reasonable doubt," whereas in a civil case, most states merely require a "preponderance of evidence." It is up to the judge or jury to determine if either burden of proof has been met.