Hi guys. I saw many threads where people have felt how Dev reached the conclusion. This is the way I have analyzed it. As I have been observing this character quite closely from the b'day cake issue.
From the time Durga bought the b'day cake Dev was in confusion as so far Durga justified everything with the diary. Also he couldn't confirm whether something is in diary or not as some pages were torn of. But b'day cake issue he was sure was not written in diary. But the crux was whether Nithya told Durga this as Nithya is a different entity till that point.
Then came the signature. I call it a blooper as when you switch identity Durga's signature has to exactly like real Durga's not this one. But if you overlook the 3 dots in the A became another similarity. Then again the FB of her wanting to sleep in sofa and not wanting to have food alone was shown. It was a totally confused Dev who came to court.
During the entire court proceeding Dev was observing her. Thinking she is so much like Nithya, to the extent he murmured in mind is Nithya hiding in Durga or Durga is hiding in Nithya.
For me it appeared he has somehow at that point itself reached the conclusion who she is. But he was looking for confirmation. I mean heart was saying Durga is Nithya, but head was not agreeing to the possibility completely.
He didn't confront her at that point instead went about trying to collect proof of the accident and what happened in that accident. Presence of death certificate brought out the contradictions. The way Durga cares Payal, letter, cake and signature. At that point as far as I was concerned he knew who actually Durga is. That's why he was more disturbed and asked Durga whether Nithya had forgiven him only for namesake. He did a small confrontation to gauge her, again she avoided questions related to her identity and told him not to think so much.
Once he got the car number and relation of that to Dr Dayal Thakur he only needed confirmation to his theory. That's why he went and told Dr Thakur you caused her accident and she died. Dr Thakur firmly told him Nithya is not dead and she is alive and she will come in front of him on Nithya's b'day. That means that death certificate has been fake and kept to mislead anyone who comes in search of Nithya. Secondly Dev had gauged Dr Dayal as a person who has some integrity. That's why without implicity telling he told he knows where Dr Dayal has hidden his Nithya.She was always there with him and he was thankful to Dr Dayal for bringing her back in his life. Dr Dayal remained silent. He didn't tell a no to Dev's theory. He didn't tell you are wrong strongly.
Actually if Dayal had wanted he could have done that. Because his firm no to whatever Dev told would have placed a confusion back in Dev's mind. But remaining silent he gave a mute acceptance to Dev's theory in a way he could tell Durga that he didn't tell Dev anything. Would say as a father Dayal wanted Durga to have a life with Dev. You can say his own selfishness as far as this part was concerned. Dev made his job easier as himself ran around collecting proofs.
Now Dev was convinced and in his car he calls him not only dumb but also tells he ignored his heart's voice and was searching for proofs. His heart had already directed him towards Durga as his Nithya. We all know Dev had more than platonic feelings for Durga even during jealousy track and it was very much evident post jealousy track.
Durga was cornered the moment Dev called her Nithya. If Dayal had informed her she would have taken some precaution to deal with the situation. But Dr Dayal was selfish here as a father and he decided that let Dev and Durga deal with the issue.
Sorry for the long winding post. But this is how I see it.