Similar to other fuels, biodiesel quality specifications have been created by the ASTM (originally known as the American Society for Testing and Materials).  Consumers of biodiesel meeting the ASTM specification can have confidence that the fuel will perform as intended.  Fuel producers must meet these specifications in order to be eligible for federal tax credits and to participate in the Renewable Fuel Standard.    A good summary of the B100 specification (100% biodiesel) can be found at this National Biodiesel Board page:
 www.biodiesel.org/pdf_files/fuelfactsheets/Biodiesel_Specification_Nov2008.pdf
A number of these specifications are specifically designed to monitor whether or not the biodiesel producer has extracted the catalyst from the biodiesel without introducing water or any other contaminant.