There's an easier way:
First put your egg in a box next to a different Pokémon. E.g., I put my egg into my Electric Box, which is next to my Fire box. The top left in my fire box is Magmortar so I put the egg in the electric box in the top left. OK, go to Magmortar, and wait. It'll show "Fire" and then you count to 5 and then press l/r whichever way that box is. So I press "r" as my electric box is to the right. The nature of the egg will flash, and then go away. Sometimes, you'll also be able to see what type it is. Like Ghost/Poison for Gastly or what not.