Well, perhaps i watched too many detective movies and know to many books of this kind^^.
But here is how you basically can guess the criminal in a lot of movies/dramas (i just want to show, that it's not magic or some tricky thing):
1. First of all in movies and books, the criminal is always one of the characters, who are shown more than just once or twice. So in the case of WR, there are ~10-15 of such characters+the teammates. So these are all possible suspects. True, it's not always the case, but in 80% (there are also cases, where are Jack the Ripper-like criminals, who are not revealed, even after the ending).
2. If there is one "main victim", then
a) then you can be sure, that the description 1. is true.
b)the criminal is one of the characters, who are close to the victim (friends, family, colleagues, employees, employer). So u can basically exclude all the characters, who had nothing to do with the victim prior the story.
In the case of WR you can exclude the female lead, her father, her brother, her friend and her boss, well and also the victim. This leaves Jin Dong Soo + wife(+child...well unlikely^^), the manager, the ahjuma, the reporter, the ex-gf and the teammates.
So now comes the guesswork, where you can make the most mistakes. Until now it's more or less pure reasoning.
3. In a lot of detective movies you can exclude the characters, who seem too guilty from the beginning and have in the beginning the strongest motive. These are in most cases red herrings.
In case of WR you can scratch Jin Dong Soo and the reporter. That leaves the ahjuma, ex-gf, Jin Dong Soo's wife, the manager and the teammates.
4. You take the crime and think, who couldn't possible (or unlikely) do this.
After the soup-incident you can exclude the ex-gf, because she couldn't do this without being noticed. Also the wife is unlikely, since a woman would be noticed in this environment (ok she could have visited her husband...but she would still be noticed, well or visit the victim at home). She is still a possible criminal, but an unlikely. The teammates are too "bland" in this drama, so it's also unlikely, that it was one of them. So this leaves just the manager and the ahjuma as the most likely suspects.
5. At this point you have to watch out for possible backdoors and the background-stories of the characters (well not in WR, but in general). You have to look, if the writer introduced some accidents, familydeaths or some surprising connections of the victim and some other character, who seemingly had nothing to do with the victim (on the surface). These could be red herrings but also the solution for the crime.
True, with these reasonings you can't solve every drama/book, but in most of the simpler crime-movies/books it hits the spot right away. The good books and movies will avoid the points i wrote and let the reader/viewer guess till the end (but also provide hints, without being too obvious).[/spoiler]