Also, only transitive verbs have a "true" or direct passive form. Intransitive forms only have an "affected" (I learned it as adversity, 迷惑, or 被害) passive.
And, as jonny-mt mentioned, if there is an を it's most likely going to be the adversity passive, unless the dative-marked noun of a ditransitive verb becomes the subject of a direct passive. I'll give an example to help you trod through the terminology:
Ditransitive means that a verb takes two objects: one direct, and one indirect. So, verbs of giving, receiving, and placement are ditransitive verbs. E.g. あげる, 与える, 置く, 入れる, etc. The dative-marked noun (or noun phrase) is the one that is marked with に, so in the sentence 校長先生が学生に賞を与えた, 学生 is dative, or indirect object. Now, this sentence has two passive forms: the first is probably more common, but don't quote me on that -- (校長先生に(よって))学生に賞が与えられた. Note that you can leave out the original subject, 校長先生, because one of the reasons you make a sentence passive is to avoid saying who's doing the action, usually because it's not important.
The other passive version is 学生が賞を与えられた, in which the dative noun (phrase), i.e., the one marked with に, becomes the subject. In this case, the object of the original sentence maintains its object marking (賞を), and there is no adversity meaning. This sentence simply states that the students were given a prize as a neutral fact, and doesn't have the meaning that they somehow suffered from it. I hope that was clear, and not too confusing.