A legality checker is not necessary. The Eggs are always legal. Even if the parents are hacked, modified, etc. the baby will be legal. The only time that an illegal baby is possible is if someone hacked the Egg itself, which is unlikely.
The reason for this is that parents don't pass down any of their hacked traits, because the game isn't supposed to. If someone hacked a Pokemon's stats to 999, that wouldn't be passed down, only IVs and natures can be passed down. If a parent knows a move it shouldn't have, then that does not make the offspring illegal either; the offspring will only receive the illegal move if it is a move it can learn legally via level up, Egg move or other means. If it can't learn the move, it won't be passed down to it.
The only thing that should be of concern is if someone used PokeEdit, for example, to give the parents perfect IVs and a good nature, then that could potentially cause the baby to have those traits. However, there technically isn't anything illegal about them besides how they were obtained; the baby could get those stats and natures with luck, so really all it does is improve your odds.
So basically, because of what the game passes down to the baby, it's impossible for any major illegal traits of the parents to be passed down. The only time it'd be an issue is if someone hacked or modified the Egg itself, which is so unlikely it isn't really worth considering.
