If you plan to produce your own biodiesel you should strongly consider doing some quality test before you consume the fuel.  When vegetable oil is converted to biodeisel there are many factors that must be right in order for the conversion to be successful.  If this conversion is not successful and a complete reaction is not acheived then fuel quality will be very poor and you will risk engine damage.  The 3/27 test is used to check for unreacted mono, di, and triglycerides which are known to cause buildups in diesel engines.  The 3/27 will give you an idea if your fuel would meet the ASTM spec as the ASTM testing is usually way to expensive for the homebrewer.  

The first step in completeing the 3/27 test is to measure 27ml of methanol into a beaker.  Next measure 3ml of finished biodiesel and place it in the beaker with the methanol.  Next, you should mix this two ingredients throughly.  After mixing look to see if all of the biodiesel dissolved in the methanol if it did then your fuel passes the 3/27 test.  However if the biodiesel does not completely dissolve and some of it seperates out then the fuel failed the test and should be rereacted.

The 3/27 test is an accurate measure of how well you converted the vegetable into biodiesel.  Therefore if you failed the test then you did not completely convert all the vegetable oil into biodiesel.