Visible crew/equipment: When Nisi and Mickey first walk into the bathroom Mickey asks why there are two toilets, and in the next shot the microphone pops up into view at the bottom of the screen.
Other mistake: While Nisi and Mickey are in the kitchen, when Isaac and Manley walk in, on the counter is a box of instant grits with tape covering its brand name (Quaker) and a bit of tape covering the face of the company logo. Then when Mr. Blakemore takes out the ice cream there's tape covering its brand name as well.
Answer: He knew right away, because he knew that Lily didn't have any children, so Nisi and Mickey could not be Lily's granddaughters.