I can't really tell myself when I am to identify them but active voice describes a sentence where the subject performs the action stated by the verb and in passive voice sentences, the subject is acted upon by the verb.
Some examples:
"The man must have eaten five hamburgers." This one is active because the 'man' is the subject and it is doing the 'eating' which is the verb.
"Five hamburgers must have been eaten by the man." This one is passive since 'hamburgers' is the subject and are being 'eaten' which is the verb.
Does this make sense?