[header] How to help an indoor cat being tormented by an outdoor cat [title] Look for signs your cat is upset. [step] It should be fairly obvious if your cat is upset. It may hiss or yowl. 
It might also puff up its fur, a tactic designed to make it look bigger and more menacing to other animals. It may also become more aggressive, attacking you or other family members (human or animal) because it can't take out its aggression on the outdoor cat.