A first fuel and a second fuel are marked with a marker that can be detected quantitatively in a predetermined concentration range. The second fuel is marked with a binary marker. Decreased concentration of the quantitative marker, presence of a binary marker, or both may be indicative of a fuel that is altered (e.g., mixed, laundered, diluted, or adulterated). Testing a fuel includes testing the fuel for a presence of a first marker in the fuel in a predetermined concentration range, and testing the fuel for a presence of a second marker. The presence of the first marker in the predetermined concentration range and an absence of the second marker may be indicative that the fuel is unaltered. The presence of the first marker in the fuel in a concentration less than the predetermined concentration range or the presence of the second marker may be indicative that the fuel is altered.