Continuity mistake: When Ghostface kills Dewey and kicks him over to the ground, it cuts to Ghostface, then back to Dewey, and suddenly Dewey's body is in a slightly different position, for example, his hands go from pointed down to pointed out, and he has moved about 1-2' closer to Ghostface. (You can use the blood puddle for reference.) A few moments later, he's back in the original position and distance. The amount of blood around his head on the floor also changes repeatedly between shots as he's lying dead.
Continuity mistake: After Dewey shoots Ghostface through the cabinet, when he walks back over, Ghostface is in a completely different position and the cabinet is broken differently than before. True, Ghostface wasn't dead. But we didn't hear any movement, and it would have been impossible for so much change to have happened without it being audible.
Continuity mistake: When Dewey shoots at Ghostface after the elevator door opens, Tara immediately starts to curl into a fetal position for protection, but then it cuts, and she's no longer in that position.
Continuity mistake: When Ghostface is talking to Sam on the phone at the hospital, there's a prominent closeup of his hand holding the knife, and it's pointed right towards the wall. The camera cuts, and suddenly he's holding it pointed left towards himself. This mistake happens a second time a moment later.
Continuity mistake: When Ghostface tells Sam, "maybe you're too weak for this franchise," on the phone at the hospital, the way Tara is crawling away from him changes instantly between cuts. (She's suddenly a bit closer to the floor and on top of her hands that were still in front of her before the cut.)
Continuity mistake: When Gale tells Dewey she came because she was worried about him, he turns his head to his right. It then cuts and suddenly he's looking forward at Gale again.
Continuity mistake: In the final two shots, we see Judy's face. The amount of blood on it greatly increases between shots. True, she was off-frame for about two seconds, but she wasn't breathing or moving enough to suddenly make so much more blood appear on her face.
Continuity mistake: When Judy Hicks is being stabbed to death, in the shots facing up at Ghostface, you can tell she has her head tilted up off the ground. But in the shots facing her, her head is always on the ground.
Continuity mistake: When Wes accidentally scares his mum, in the first shot, he has a pair of white wireless earbuds in his ears. But then, the camera cuts and they're instantly in his hands. Then, the camera cuts again and his hands are suddenly about 6" higher in the air.
Continuity mistake: When Sam is driving away from the Meeks' house and has a vision of her father talking to her in her car, pay attention to the background outside the car windows. The backgrounds of the shots facing forward down the street and the side-shot facing Sam do not match in the slightest. She's driving by completely different houses and trees as it cuts back and forth between the two angles.
Continuity mistake: When the group is at the Meeks' house and Chad says, "Ok mom, we're good," Liv has her hand on his shoulder for two shots before she suddenly has her elbow on his shoulder.
Continuity mistake: When Dewey joins Sam and Ritchie at the Meeks house, after Ritchie says "Just because that guy showered doesn't mean he should have a gun," he goes from facing sideways at Sam to forward at Dewey between cuts.
Continuity mistake: When Dewey is about to start texting Gale, the way he's holding his phone changes instantly when it cuts from the wide shot to the close-up of his phone.
Continuity mistake: When Dewey calls Sidney, and she's pleasantly surprised to hear from him, she sort-of wedges her phone between her cheek and shoulder so she can check her fit-bit. Before the shot cuts, her left hand is still on the phone, but instantly in the next shot, she's holding it in front of herself to check her fit-bit.
Continuity mistake: When Dewey calls Sidney, you see her jogging, and she's about to pass two people on her right, while there's a bench with a few people sitting in the distance. It then cuts to a side-shot of her feet pointed to her right, and the two people she was about to pass disappear before she comes to a stop. It then cuts back to the first shot, and the bench with the two people is suddenly much closer than it should be. (They likely cut the shots shorter to speed up the scene, but still a mistake.)
Continuity mistake: After Sam is attacked in the hospital, the cop runs into the room where Ghostface was. As the shot is about to cut, he's pointing his gun to his left. It then cuts to the opposite angle, and he's pointing his gun to his right.
Continuity mistake: When Sam gets called by Ghostface in the hospital, in the first and third shots of her answering her phone, she has both hands on her phone. But in the closeup shot in between, she's only holding it with her right hand.
Continuity mistake: When Vince is stabbed, when he pulls his head out of the car window and stands up, he is staring straight forward. It then cuts to another angle, and he is looking down.
Continuity mistake: When the friends are talking at the bar, when Wes and Mindy have their back and forth about how they're all suspects, in the shots facing Mindy, she's holding her pool stick perfectly still. But in the shots facing Wes, she's waving it back and forth rapidly. The angle it's leaning also changes position once during an edit.
Continuity mistake: When Sam and Ritchie are driving to Woodsboro, pay attention to the background outside the windows. Multiple times, the lighting goes from sunny-style lighting to darker, overcast-style lighting between cuts.
Answer: As far as I can tell, It's never stated in the movie how he found out about that. Maybe Sam told him off screen when he "learned " that Billy Loomis was Sam's biological father.