How to check whether the Olive oil is fit for consumption or not?
Olive oil doesn't suddenly go rancid (unlike butter in which this process is pretty quick), but its taste deteriorates with time.  AFAIK it happens primarily due to oxidation, not bacteria or fungi, and there are no dangerous/poisonous byproducts.

I have a bottle of olive oil which I opened about a year ago and it tastes noticeably different from fresh oil (not too pungent to throw it out though).