In Frankenstein, what details does Frankenstein mention that make it believable that he could have created a living creature?
When Victor breathes life into the creature, technically, he becomes a father. And, typically, he does not understand his child. In fact he is revolted by the creature's appearance at the same time he is overwhelmed at what he was able to accomplish. Owing a small bit of gratitude to the monster that Victor brought to life, he achieved a great victory in his scientific research.
Victor becomes terrified of the creature, not realizing that he is fueling his anger and desire for vengeance. Promising him a mate, he then gets rid of the female parts and disregards the creature's needs only making him feel more like an outsider and hating humanity with a greater passion.
The fact that the monster has such strong feelings is proof enough that he created a living creature. The monster is a complicated personality with unique needs just like anyone else. When Victor rejects him, he goes crazy like a raging teenager rebelling against his father.
Rejected and alone, the monster is depressed and feels abandoned by his creator. Through his indifference to the creature that he gave life to, Victor seals his own fate, to feel alone, isolated and emotionally abandoned.