If given a blended product we could tell you whether it is B5, B20, etc. The amount of testing you could do on any fuel can become very extensive (and expensive) and therefore you really need to be testing for a particular reason. If a fuel meets the Australian Specification then it should be OK for use. In regard to oxidised biodiesel in a blend, there is a test for oxidation stability of diesel fuel, but it is really only going to tell you that the fuel blend itself passes or fails. It may be possible to do some analysis on the oxidation residue to determine whether it was from the fossil fuel or biodiesel, but again it could get quite involved and expensive. Significant levels of third party contaminates should show up in the specification tests.
Do you use oxidation inhibitors in your home produced products.

Steve