1) By accident - she overhears someone talking (maybe Inder and Barka or Barka and Nani?)
2) Barka says something in her anger while Nikhil is defending Lajjo
3) Ayesha says something in her anger at Laajo when Nikhil tries to interfere
4) Ayesha with her big mouth has already told Sid everything (remember yesterday's advice Barka was giving about not sharing every small detail with her future husband and in-laws) and he spills the beans
5) Laajo pieces things together overtime watching the interactions between Nikhil and Barka and realizes the attitude is more than just LaKhil getting married
6) Baba or Bhua tell Laajo
7) Nikhil tells her during a heart to heart talk with Laajo.
8) Laajo asks Gunja for help after her #5 in the listđ as she needs help with the puzzle...
9) Meera tells her as she wants to her to understand Nikhil and try to make a place for herself in his home (as she has been unable to do in her own house)
10) Some other means...
Please add if you can think of anymore ways...đđđ