* Past Tense Verb: When "rented" is used to describe an action that happened in the past, it's a verb. For example: "She rented a car for the weekend."
* Past Participle: When "rented" is used as an adjective to describe something that has been rented, it's a past participle. For example: "They live in a rented apartment."
So, you need to look at the sentence to determine the part of speech.