That is the most unrealistic way to find something as such out.
Payal has not lived with Shubh for that long. There are things and stories that only Aastha would know and not Payal. There are conversations or romantic and intimate moments that only Aastha has shared with Shubh and not Payal. He should take them into the room one by one and ask both of them questions that only the real Aastha would know. I think the real one is the one with the saree as she is the one who has those dunken swollen eyes like she hasn't rested in super long.
This serial is really becoming very irritating now.