Short answer: using logic and reasoning aka my brain. I pay close attention to character development and dynamics plus the story's overall plot.
Long answer: if you pay close attention to the story, it's actually very obvious that Hwaryun and Suu will be endgame if there's romance in the ending. Their interactions throughout the story strongly support this. Just go read through the chapters and pay close attention to the characters. Ch.49 is a really good example. Another one is the most recent chapter, Ch.141. Other scenes I can name off of the top of my head are: rice cake millets, Suu's reaction after finding out about Hwaryun's past, the lantern festival, etc. My point is: it's pretty clear that Suu and Hwaryun love each other and it's not hard to figure this out. Their situations and circumstances, however, make it difficult for them to be exclusive.
Also, Hwaryun is also the male lead in this story. If you're really interested in why I say this, then go ahead and look for my other comments to other posts/ read my own posts about Hwaryun; they're available in the topics section of this story and my profile. I've explained enough about this topic in great detail.

who's the endgame?