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.
Continuity mistake: When Liv says to Chad, "Aw, you gonna stalk me like a jealous boyfriend?" outside the school, her head goes from tilted 90 degrees to the left to suddenly being tilted about 45 degrees to the right between cuts.
Continuity mistake: When Chad and Liv are talking about how he sent her a "Find My Fam" request outside the school, Chad starts to turn his head from Liv and towards Wes and Amber. Then, it cuts to a close-up of him, and he is suddenly looking at Liv again and turns his head towards Wes and Amber a second time.
Audio problem: When Ritchie tells Sam, "I'm coming with you," right before they leave the bowling alley, Ritchie's line seems strangely dubbed in. It just doesn't quite match his lip movements for the first half.
Continuity mistake: When Sam and Ritchie are outside the bowling alley and Sam says, "I gotta go!" after getting the call from Wes, suddenly she has her hands up by her face instantly between cuts, right before Ritchie stops her. (It happens quickly, and she's facing away from the camera, so it can be hard to catch.)
Continuity mistake: When Sam's phone rings outside the bowling alley, and she pulls it out, the way she's holding it changes completely between shots.
Continuity mistake: After Ritchie and Sam kiss outside the bowling alley, watch closely. Sam goes from having her left arm down below frame to up touching her chin instantly between cuts.
Continuity mistake: When Ritchie approaches Sam outside the bowling alley and jokingly asks her what she just took, when the camera cuts from one angle to the next as Sam replies, "Your boner pills," the look on her face changes completely between cuts. Additionally, in the first shot, she was holding up an energy drink that should be visible in the second shot, but isn't.
Continuity mistake: In the opening scene, when Tara mentions how she saw "Stab" once when she was a kid and the caller says "You live in Woodsboro and you don't know 'Stab?!'", the way Tara is holding the phone changes slightly between edits.
Continuity mistake: In the opening scene, when Tara puts the mail down on the counter and stands next to it while on the phone, the position of her body and the mail changes slightly when it cuts from the medium shot to the wide shot.
Continuity mistake: When Tara is in the hospital and is about to (accidentally) hit Ritchie with the phone, the way she's holding the phone changes between cuts. In some shots, she's holding it right by her head; in others, she's holding it above her head.
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.