The problem with Biodiesel is that it is unregulated and that it may contain impurities. So much so that big daddies like Mercedes-Benz, say that using anything greater than B5 (five percent biodiesel/95 percent petroleum diesel) will void the vehicle's warranty. They say that the impurities in the fuel could clog the injectors. To circumvent this catch-22 situation, where one part of you says that you should go green and the other says it's not yet practical to do so, is the i-Spec Q-100 from Paradigm Sensors. This handheld gadget allows you to analyze biodiesel fuel quality.

It tells you the percentage of biodiesel to petroleum diesel. It also shows acid and methanol levels.

Source