Fred 2: Night of the Living Fred

Fred goes into music class to find his favourite teacher, Mrs. Felson, has been replaced by a teacher named Mr. Devlin. Walking home from school, Fred notices a strange girl named Talia following him. Fred believes he is being stalked, but the girl just walks the same way to school as him. Spying on Devlin that night, Fred sees him burying something, which he suspects is the body of Mrs. Felson. Kevin's mother invites Fred and his mother to a party for Mr. Devlin. At the party, Fred's mother falls for Mr. Devlin, and Fred learns that Talia is actually Kevin's sister.
The next day, Fred becomes extremely suspicious of Devlin and reaches the conclusion that he is a vampire. Mr. Devlin takes Fred's mother on a date, so Fred enlists his friend Bertha to spy on them. At a restaurant, Fred and Bertha learn that Mr. Devlin doesn't like fries, making them all the more suspicious. Later that night, Fred's imaginary Dad brings him to a wrestling arena, where they tag team against Mr. Devlin and Kevin.
Fred is horrified at school the following day to discover Mr. Devlin running a blood drive at school. He's even more worried to discover Bertha taking personal music lessons from Devlin. So Fred gathers items to use as weapons against Devlin. He plans to defeat Devlin at the school piano recital, where Mr. Devlin will be performing.
Fred arms himself with various tools he's gathered up for the big vampire eradication. He then goes on a shooting spree at the recital, soaking everyone except Devlin. Mr. Devlin invites him into his house for dinner after the fiasco, so that they can bury the hatchet. Fred sets up a live video streaming from his cell phone so he can prove to his classmates that Devlin is a vampire. After showing off Devlin's living room and digging for "bodies", Fred discovers a secret butchers' room behind a wall filled with meat and bones. As Fred investigates, Devlin creeps in with a long knife and a tall head-dress, Fred then drops his phone into a pot of boiling liquid, freezing on Devlin, and the frozen image of Devlin in his headdress causes everyone watching the video to think that he is a vampire. But after Devlin explains every weird hobby he has, Fred starts to relate to him, and he comes to realize Devlin is not a vampire, just an eccentric and cultural music teacher.
Everyone at Fred's school has seen his video, which makes everyone believe that Devlin is a vampire. Fred runs to Devlin's house to apologize, but Devlin is too depressed to answer. When Fred learns Devlin has gotten fired and put up a "for sale" sign for his house, he feels guilty and decides to fix things by making everyone think he himself is the vampire. Bertha and Talia both help, as Talia reveals how much she dislikes her brother. Kevin and his friends go up to Devlin's house because they think he has kidnapped Talia. Fred walks out of the garage carrying Talia, threatening to turn her into a vampire. Bertha makes Kevin stab Fred, and he gets sprayed with fake blood, making everyone believe Fred really is a vampire. Devlin accepts Fred's apology, revealing that he quit his job and sold his house of his own choice, but still sees Fred as his one true friend. As Fred's mother and Devlin go out for a good-bye snack, Fred looks at the mirror in his house and sees that Mr. Devlin has no reflection, revealing that he was a vampire after all.

Question: What reveals that Devlin is really a vampire?
Answer: looking in the mirror