An olive oil is extra virgin if it satisfies certain quality terms, which are associated with the chemical composition of the olive oil, like
- the content of free fatty acids needs to be below 0,8 %
- the peroxide value needs to be below 20

These terms are only satisfied if (among others)
- the time between the harvesting of the olives and their pressing is limited to the minimum
- the harvesting and transporting of the olives is done with care
- the olives are not overripe when harvested nor damaged by frost or hail
- the olive oil is correctly preserved after pressing (e.g. no excessive oxidation)

Furthermore, additional tests are executed to verify whether the olive oil is not mixed with refined olive oil or oils of other crops. These tests can also indicate whether the olive oil is obtained in a non-mechanical manner (e.g. chemical extraction) or whether it is rancid. According to the regulation of the European Community, some of these test results need to be mentioned on the label:
- the wax content
- the absorption of the ultra-violet light during its passage through the olive oil (indicated by K270, K232 and K)

You can find more information in this article of the Australian government (PDF).