What is the purity test for Olive oil?
Any real test for purity in olive oil is going to require a lab (or at least lab equipment and supplies). The cost of that testing is going to far exceed the price of your bottle of oil.

For home use, then:


Buy from someone reputable. Unfortunately, even reputable folks sometimes get conned by their suppliers. (If you have a local farmer you can buy from directly, that risk can be avoided).
Taste it. Unfortunately, you'll probably only be able to notice bad fakes (even experts are fooled). As a side benefit, though, you'll also get to notice poor-quality (but genuine) olive oil. 


A few countries have introduced labeling for olive oil that has been tested by an approved laboratory; if yours is one of them, you could look for certified oils.