If you are so keen on buying knock off Oakleys, you might as well make a good job of it.
You could also open the pair of knock off Oakley and keep it on a level surface to check out the balance. Wear the set and check the lens out. Make sure the shade (whatever colour you choose) is uniform and there are no splotches of colour forming on the lens.

Once you are convinced about it, there’s nothing much you can do expect buying it.
One of the most common differences between knock off Oakleys and the real ones is the latter’s capability to filter out UV rays. This is one element, which the knock off Oakleys across the board lack. So do not be surprised if the store keeper admits to the same.
But then for the cheap prices that they go by, this is small compromise you make for a deal as amazing as the ones you get on knock off Oakleys.