Final answer:
Thomas finds out that Blacula is a vampire by noticing several traditional vampire characteristics like a lack of reflection, extraordinary strength and speed, absence during daylight, and the presence of fangs.
Step-by-step explanation:
In the movie, Thomas discovers that Blacula is a vampire through a series of unsettling events. At first, he is skeptical about the existence of supernatural beings. However, upon witnessing Blacula's aversion to mirrors (which is a common trait of vampires as they do not have reflections), along with the abrupt disappearance of several people associated with Blacula, Thomas starts to suspect Blacula's true nature.
His suspicions are further confirmed when he sees Blacula's fangs and observes his extreme strength and speed, which are beyond human capacities. Finally, the fact that Blacula doesn't appear during daylight - a well known weakness of vampires - solidifies Thomas's belief that Blacula is indeed a vampire.
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