Honestly, I don't think that there is any real authority on what constitutes a complete or incomplete art set. Generally, I'd follow what is shown on the "Cart Pics" section of this website.
However, sometimes SNKP will ship additional items to different retailers. Take snk-capcom.com for instance, who received mini marquees for games like Metal Slug 4, Metal Slug 5, Samurai Shodown V. Some might take the position that the initial runs of these art sets didn't include a mini marquee, and say that the set is complete without them, while others might say that the set is only complete with the mini. Also, some games from one retailer may include both Spanish and English art sets, while another retailer might include only the English or Spanish set.
Like most things related to the Neo, there's a bit more here than meets the eye. You really have to dig to find out what constitutes a complete art set for a particular title - there's no easy way out, as far as I know. On the other hand, you could just forego all the neurotic collector shit and just enjoy the game.
