Well if you're looking for a specific character there is a way to tell: there's a small hole in the top corner of the bag (at least all of the ones I've bought do) so all you have to do is shift the card over to the hole and you can see the cutie mark of the Pony you're going to get which is printed in the same corner, then for Applejack, Rarity and Pinkie Pie you can shift the figure to the hole to see if it's the regular glitter one or the metallic variant
And there you are! You'll be able to buy them all without getting a duplicate ever again (-^u^-)