Griever

[Serapy]

Yes, there's a way to determine.

For example:

Matron gained powers off Ultimecia.

Matron begun to know about Ellone after she came to the orphanage (She learnt about Ellone).
Which explains why Ellone left the orphanage earlier because she was scared of Matron due to her powers that were recieved by Ultimecia. Proof: you see little Squall calling Sis... at the orphanage for many times, which indicated that Ellone left him/the orphanage in the first place.

Ellone didn't leave the orphanage because she was afraid of Edea. Edea sent Ellone away, to be protected by the White SeeDs, so that if she was possessed, Ellone would be out of harms way. But the gist of what you're saying is still the same.

Ellone leaving on purpose and Matron sending her away is the same difference as leaving the orphanage alone. If Matron has never gained powers in the first place, then she didn't have to leave so early.
Matron is a nice woman, I'm sure she wouldn't do the wrong thing (keeping Ellone at all the time), but it's too late for that. It doesn't even matter if Ellone is near Edea or not because Edea already have gained the knowledge of Ellone.

Later in the story, Ultimecia possessed Edea.

While possessing Edea, Ultimecia has just found out about Ellone and her special power.

Ultimecia begun looking for Ellone.

So, based on that logic:

Rinoa learnt about Griever.

Ultimecia possessed Rinoa. While possessing Rinoa, Ultimecia has just found out about Griever.

In the end, she summoned Griever.

The point is that if Rinoa has never learnt about Griever in the first place, there is no way that Ultimecia could summon Griever. Ultimecia can't just gain knowledge, unless she possesses someone.

But it doesn't determine whether Griever is a real GF, like Ifrit/Bahamut etc. which Ultimecia found, or whether she summoned it out of Squall's mind. I feel the former is more likely, but they both have merits.

Ultimecia may have found out about Griever from Rinoa when she possessed her. Or, perhaps Ultimecia read it out of a history book, or a book on GFs, or Squall's autobiography. One cannot prove that one way or another either, although it is definitely probable that Ultimecia found out through Rinoa.

But, I don't know how that shows Rinoa is Ultimecia.

During Rinoa's conservation with Squall about the ring, you will get the chance to name it (default is Griever). You also get the same chance to name other GFs, which probably show that Griever was a GF after all (due to the name choosing). But what's the point of having an non-existent GF in the whole game (apart from Ultimecia summoning it) and why did you have to name Griever when it's part of the story and not gameplay? GFs are meant to assist in battles controlled by your characters, Griever certainly didn't assist that job. Holding Griever didn't make Squall stronger either, in fact he's almost emotional at all the time. So that may rise a possibility that the main purpose of Griever is to be awaken until Ultimecia has summoned it, basically.

Or there's another possibility that Square Enix wanted to make Griever as the final GF for game play purposes but then they have changed mind, so they have put Griever into the final battle. This possibility doesn't seem to be likely, but who knows?