I know this is crazy, but you can taste it :-) Of course, most people don't know what olive oil should taste like, but it really has a wide flavour range. Buy a selection of small bottles and simply taste them (possibly with some nice bread and a little salt if you don't like drinking oil straight up).

I highly recommend trying olive oil from other countries as well. It's really an eye opener. Once you get some experience it's pretty easy to spot good olive oil. It's a bit like beer. Some people love American lite beer (and more power to them), but if you compare an all malt lager with a single variety noble hop to a light beer with 30% rice/corn and a mishmash of 8 nondescript hops (or isomerised hop extract) it doesn't require a discerning palette to detect the difference.

Interestingly, I have been wondering for about 10 years now about how ridiculously cheap balsamic vinegar has shown up in the super market. Buy a bottle of 30 year old balsamico and compare it to the cheap stuff and I suspect you will wonder (like me), "Is this really balsamic vinegar???"