Final answer:
Alexander Fleming tested the hypothesis of how penicillin was discovered by conducting experiments. He observed that a mold, later identified as Penicillium, was able to kill bacteria and confirmed this through further experiments.
Step-by-step explanation:
The hypothesis of how penicillin was discovered was tested by Alexander Fleming through a series of observations and experiments. He noticed that a mold, later identified as Penicillium, was able to kill bacteria. He conducted further experiments by growing Penicillium in broth and then pouring the broth into culture dishes containing bacteria. The broth was found to kill the bacteria, confirming the hypothesis that the mold possessed antibacterial properties.