How to know olive oil has gone bad? What are the signs olive oil has gone bad?

The simplest idea is to check the bottling date and best-before date. However, if you doubt the oil has gone bad, even if the best-before date has not passed, a simple sensory test, aka sniffing, at home can provide some clue.

The first sign of an olive oil going bad is the presence of a rancid odor (wax or crayon-like smell). Diminish in fruity note that signifies the freshness of olive oil is also noted.  

Analytically, free acidity greater than 0.8% and peroxide values above 20 meq O₂/ Kg signify olive oil has gone bad.