I think you're just supposed to deduce it from the fact that... touching the lantern as Shogo did something. I guess another clue was that Yoko said Shogo was a spirit adept.

Throughout the game it is implied that he has some real mojo. The clue that led me to it, is that Mio says something akin to "It would be someone with a great sprit sense, too bad we don't know anyone like that..." Consider that Mio has met literally every character except Shogo and is still drawing a blank, it really only leaves one option.

Well its a bit much and maybe im reaching, but my line of thought would be this:
1) Shogo killed Yoko by using the whispering canal against her
2) we know he is the original owner of the beckoning light because in the Majority timeline, Shogo dies, Takumi is nearby and discovers the body with the curse stone and leaves it in Mayu's hostage room
2a) Yoko left the body alone because a) he probably only gave her a charge of 1, indicating he is just a normal guy and b) he was acting confused towards her indirect poking. Those 2 things combined made it clear to her that he was just an innocent bystander
3) We know the abilities of The Beckoning Light thanks to Mayu.
4) we later learn that the story of the beckoning light is about Seiman

Originally posted by QuJee:

Well its a bit much and maybe im reaching, but my line of thought would be this:
1) Shogo killed Yoko by using the whispering canal against her
2) we know he is the original owner of the beckoning light because in the Majority timeline, Shogo dies, Takumi is nearby and discovers the body with the curse stone and leaves it in Mayu's hostage room
2a) Yoko left the body alone because a) he probably only gave her a charge of 1, indicating he is just a normal guy and b) he was acting confused towards her indirect poking. Those 2 things combined made it clear to her that he was just an innocent bystander
3) We know the abilities of The Beckoning Light thanks to Mayu.
4) we later learn that the story of the beckoning light is about Seiman

2) Why would Takumi just leave a curse on Mayu? And it is implicit in the story that only those who already have the curse can receive a curse, so Mayu was already a bearer of Beckoning Light. In other words, Shogo had not received the curse in the story where he didn't "believe" the legends.

After getting the yin scroll info about how to gather all pieces of Seimen's cleansing ritual, the one main key thing to think about how Shogo is linked to Seimen is the Beckoning Light's curse echo having the ability to use curses against other curse bearers. If you think about the circumstances:
- How the Whispering Canal was next to Yoko's dead body
- How the Beckoning Light uses the other curse stones' curses against their bearers
- How similar Shogo's death is compared to Yoko and the people he killed to gather soul dregs
- How Yoko was telling Shogo was telling him to look over to where she was pointing, and suddenly dies after you pull the curse trigger and you turn around to witness her death

It pretty much hints Shogo is Seimen's descendant, with you (Seimen's soul) laying dormant before you begin to possess Shogo after the counter-curse activates.

I guessed it because Storyteller tells Shogo killed no people, so he had to be possesed.