The five ways to identify the quality of the oil are seeing, smelling, hearing, tasting, and inquiring. First of all, pure vegetable oil is colorless and transparent,and if added by some pigment or impurity during production process, its color and transparency will be changed.
Secondly, each type of oil has its unique smell. If there is a slight foul smell coming out, it might be the "gutter oil" because the normal cooking oil will only send out slightly faint scent. And it is more likely to be "gutter oil" if it smells stinky after being heated.
Moreover, you may taste the oil using chopsticks. If it has slight sour or strange taste, it probably is the “gutter oil”
In addition, you may identify the sound when you light up the oil. It will be the qualified oil if there is no sound coming out. If it is with “creak” sound and abnormal combustion, the water content in the oil must exceed the standard. And if it has explosion sound, the oil must contain much higher water and probably mixed with fake products.