As far as the title question is concerned, I think these are the reasons for Sai believing Pulkit without verification:
1) Pulkit did not approach her nor is it widely known in college that she is from Chavan family.
2) The facts that came in front of her from Pulkit’s side tied in logically with the facts she managed to get out of Devi tai. One side of the story perfectly completed the other side like in a puzzle.
3) Pulkit is clearly in a respectable position & has no apparent motive to deceive Sai or Devi.
4) His open warm sincere manner.
5) Devi is too innocent to make up such wild stories, that too exactly tying up with Pulkit’s.
6) Later, Kaku & co’s own reluctant admissions also matched the story she had found out.
I have only one crib - she cld hv called Pulkit and got the lambi kahaani of Harini out of him, so that she has the full picture, before embarking on mission Devkit.
And, she cld have taken Virat into confidence beforehand, citing the 6 points above to support her case.