I feel until and unless she sees Sona speaking to Ronit her doubt will not turn into SURETY. Because,as per the show except Ronit, not a single person knows Sona's true identity. So, all of them is behaving normally with Sona and Sona also is basically wearing a mask while conversing with all of them. And thus,seeing Sona with all of them Prerna might doubt herself actually thinking that Sona can't be Komo as Komo wouldn't have behaved so nicely with all others.
But, it's only Ronit in front of whom Sona's mask falls and she speaks in her own Komo style. Hence, I think once Prerna sees Sona speaking with Ronit that too in her typical Komo style, it shall lead to Prerna finally convincing herself that yes,her doubt is right, Sona is Komo. If there is any scene between Ronit and Sona, and Prerna overhears them, I am sure two questions will pop in her mind, "How does she know Ronit ? Why do I get the feeling that they are very close ?". The 2nd question might lead her to further investigate whether Sona shares any relation with Ronit or not and this might be the stepping stone to further truths being revealed.
So,as I said I have a strong guess that until and unless there is any scene between Sona and Ronit with Prerna witnessing it, there is no chance that Prerna shall actually be convinced regarding her doubt and start acting on it. Till that scene happens, Prerna's doubt shall only remain as doubt. Because the only link between Sona and her true identity is Ronit.