Locate the verb in your sentence. A verb is a word expressing action. This can be a physical action ("walk," "jump," "throw") or a mental action ("hope," "dream," "impress").
Check whether the verb is paired with a form of "to be." For instance, if the verb is "performed," is it simply "performed," or is it "was performed" or "has been performed"? If the verb is not paired with a form of "to be," the sentence is not passive.
Identify the subject of your sentence. The subject is the noun that performs the verb. For instance, if your sentence is, "Jonathan watched the football game last night," your verb is "watched." Because the noun who "watched" was Jonathan, "Jonathan" is the subject of your sentence. If your sentence does not name its subject, it is passive. For instance, in the sentence, "Native Americans were oppressed for centuries," the person or group oppressing the Native Americans is unnamed, making the sentence passive.
Determine whether the subject comes before or after the verb, if you have named the subject in your sentence. A passive sentence puts the subject after the verb. For instance, "Jonathan watched the football game" is in active voice because the subject ("Jonathan") comes before the verb ("watched"). If you had written "The football game was watched by Jonathan" instead, it would be in passive voice because the verb would precede the subject.