Look for the provenance, can you find out who is the actual producer?
If the label says "Made in the EU" that is a very worrying sign, probably even the bottlers/retailers themselves do not know where the oil comes from an in all likelihood it is at best a random mix of several oils.
Try to go for an oil from a Single Estate, with a clearly identifiable producer that can be traced and checked out.
Preferably go for a mono-cultivar oil: mixing different grapes willy-nilly is sacrilege in the wine world but it is common practice in the olive oil business, much to the detriment of the product.
Organic producers have to comply with very strict international criteria so an oil carrying the approved organic logo is a sign of a certain quality.
Also look for a production date, if the oil is older then 18/20 months do not use it
To taste it:
If you are OK with tasting olive oil in pure form just take a teaspoonful of it, put it in your mouth and then suck a bit of air in so the oxygen gets to the oil and amplifies the taste. The oil should not have unpleasant tastes and should not taste somewhat "old" or stale. As a rule of thumb the oil should have a "grassy" taste, perhaps with overtones of fruits and vegetables depending on the variety. It should also leave a burning sensation at the back of your throat. This is actually caused by the polyphenols in the oil which are the main source of health benefit associated with it.
If you find it difficult to taste oil on its own, then just dip a small bit of bread into it, ideally a simple white or wholemeal bread without too much taste of its own and look for the same features highlighted above.