Visible crew/equipment: When Julie says "It's probably just your nerves" then Heather replies "Nerves?!", above Heather's head you see a boom mic dip down into view. (00:12:25)
Visible crew/equipment: When Heather is leaving to go to her TV show, she and Julie walk out Heather's house. As they walk out and talk, you can see a boom mic dip down into view for a while. (Seen on DVD version) (00:13:25)
Visible crew/equipment: When on the phone to Chase, Heather says "He's had some sort of an ... episode" and as she says this a boom mic briefly dips into view, above her head. (Seen on DVD version) (00:23:40)
Visible crew/equipment: The boom mic dips into view once again whilst Heather is on the phone to Chase, as she says "...Like Freddy." (Seen on DVD version) (00:24:00)
Visible crew/equipment: You can the microphone dip into view whilst Dr. Heffner is whispering to her junior nurses, as you can hear Heather wrestling with the security officers. (Some DVD versions.) (01:14:00)
Visible crew/equipment: After Julie is killed, she falls to the floor, and if you look closely you can see the big black mattress she lands on. (Seen on DVD version.) (01:18:35)
Visible crew/equipment: In the playground scene, when Dylan jumps from the top of the rocket, it's an adult stunt double.
Chosen answer: Mostly just very subtle hints. In the beginning when she arrives right around the same time that Heather gets a call from the psycho-stalker, suggesting a possible connection. And the fact she's alone with Dylan many times during the film, as she could be subtly manipulating him to act out.