Griever

[champagne supernova]

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.

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.