How do you know if diesel fuel is contaminated?

Two types of analysis can determine if your fuel is contaminated:

Qualitative testing involves taking a sample and performing a standard visual screening of the fuel, according to ASTM screening standards to quickly determine if the fuel is contaminated and off-spec. 

Quantitative testing involves in-lab, ASTM-standard testing.  A variety of test methods are used to obtain definitive results for qualities such as flash point, PPM count of particulate contamination, percent of water contamination, cloud point and pour point. 

Diesel fuel contains paraffin wax so at low temperatures it clouds up, gels up and does not flow or pour well.  Contaminated diesel can cloud up in higher heat and disrupt engine performance.