Folks say to put olive oil in the fridge to see if it semi-solidifies. If it does, it's supposedly the real deal. All my olive oil samples have solidified, from store-brand all the way up to premium product. Maybe I've been lucky, or maybe the test is pointless.
EDIT: Amanda points out in the comments that it's not a good test for determining the quality of olive oil. I think that's probably true, it's the fat content that largely determines the freezing point.
Consider though if highly diluted with sunflower oil. Olive oil is mostly oleic acid (mp 13-14C in free acid form). Sunflower oil is mostly linoleic acid (mp -5C in free acid form). Triglycerides melt at lower temperatures than their component fatty acids. It seems likely that sunflower seed oil would not solidify at fridge temperatures, while olive oil may. (I haven't done the test myself, no sunflower oil on hand and not going to buy it out of curiosity.) Commercial salad dressings don't freeze at fridge temperatures, and they're vegetable oils (largely PUFAs).
If olive oil counterfeiters were smart, they'd use high oleic acid sunflower oil. Much more similar to olive oil in fatty acid composition, it might not produce a significant change in melting point profile. Not sure that's any cheaper than olive oil itself.
Then again, high oleic sunflower oil is probably nearly as healthy as olive oil.