NES carts with converters in them weigh ~4.5 oz., without ~3.5 oz. 
Cool, fair enough. There are other ways to tell, but if that works for you, awesome! Make sure to look out for 5 screws instead of the usual 3, also, carts without the clips on top likely have a converter. If you pry the cart apart slightly, not enough to damage it anyway, but just enough to get a peak inside, you can also see that there's no "legs" for a chip, which is usually a dead give away that it has a converter.