99 of out 100 times, if the MC goes out of her way to swear, "I won't fall in love with you in this life!" then you know she's going to fall in love with him. See also:
The ones who genuinely want to move on do so almost immediately. If it's been more than 20-30 chapters and she's still engaged to the bastard guy, then you know who the ML is.
And btw, I agree that whoever spoiled the ending is a hero among readers. Same with the one who spoiled Beware of the Brothers. Nowadays I always look for ML spoilers for angsty series to preserve my sanity.