She always has been quite observant, so this checks out, yeah, the questions seem calculated in order to somewhat gauge a reaction, and a lot of the answers were done with hesitancy, so she could guess something was likely off.
Shouya nearly kissing her, and the face Ritsu gave likely gave her all the confirmation she needed to get a (not concrete, but pretty solid) guess that she wasnt fucking with it, and she just went in on that basis.
She may seem like a space case, but she is surprisingly sharp in terms of people reading and intuition.
No shock she was rejected though, Ritsu has some character development to do to even muster the courage to unironically break up with him, so the lex will have to wait till then.
And I definately knew they were getting in together the moment Ritsu got dirtied lmao, what a lovely bunch.