Yes, precisely what theSLAYER said! But even then I'm not sure if it's the best way to tell or not. It might work! But I also don't know how the "As Egg" function works with these games. For example, it might just be creating a standard Egg and modifying its values to contain Manaphy inside of it--which you might be able to force even if the game wouldn't typically allow for it or have any reason to ever create. Having that egg show up with Manaphy's Egg model (if it exists) might require something like PKHex identifying Manaphy specifically and telling it to use its egg, or the games telling all regular eggs, "Hey, if there's a Manaphy in you, use THIS model" which seems a bit excEGGssive if you ask me. (Sorry not sorry~!) It's worth a try to see and find out, but even if it doesn't work to show up as a red and blue Manaphy Egg I wouldn't be quick to say that disproves its existence. I'm thinking it might be more likely that Manaphy Eggs might exist as a completely separate entry somewhere. I haven't actually cracked open this game yet myself, but I've figured that lots of other people already have or already have it up and might be able to answer my question. I was thinking that an easier way to go about it might be in something like a resource viewer where we could just see a list of all of the Pokemon models and such, find where the regular egg is in there, and if anything else might stand out? I did do a small bit of research on it, but haven't found anyone saying anything about it either way yet.
Additional question: If the "As Egg" function does not generate a Manaphy within a blue and red Manaphy Egg in gen 7, does it in gen 4 or 5 or 6?