I really like this one, for me anyway it’s the best sardines out there … however the box says it’s with “Olive Oil”.
Some other products of King Oscar are with “Extra Virgin Olive Oil”.
How can I tell the “regular” olive oil is real olive oil and not one mixed/refined with soybean oil etc.?
You probably can’t, for absolutely sure, but the ingredients don’t list soybean oil. At some point you probably have to trust the label.
BTW Extra virgin is no more “pure” than regular olive oil - we’re not talking about ordinary virgins here, lol. It’s just a different extraction method.
Take a sample to a lab and have a chemical analysis done.
In a couple of his lectures, Dr. Phinney talks about the store brand olive oil he buys, and at the time he gave the talks, he said a chemist friend had analysed samples and decided they were real olive oil. Don’t know if it still holds true for that brand, since manufacturers cut corners all the time.
I’m not sure, but doesn’t real olive oil become more solid and cloudy when cold? You could put a sample of the olive oil from the sardines in the refrigerator and see if it becomes solid. Just an idea. 
I’ve done this with different olive oil brands from the store and discovered some of them stay completely liquid, some solidify partially and others become completely solid.