Identifying the passive voice is simple. It is formed by auxiliary verb "to be" and the -N form of the verb (the "past participle" if you like that name), which, of course, may actually end in -T or -ED.
BOTH elements must be present for the sentence to be passive voice, not just the participle (which is used in the perfect) and not just "auxiliary to be" which is used in the progressive, and certainly not just plain "to be" all by itself as the main verb.
And Dan T., I'm not sure how correct this is, but the way I've been checking whether a sentence is passive is by adding "by John" (or whomever) to it and seeing if it still makes sense. So, "The cake was made" becomes "The cake was made by John", so I think it's in the passive voice, but "There are salt and pepper shakers that have come into the house over the years" doesn't work. I have no idea if this is correct, but I think it's worked every time I've used it so far.
On checking my copy of Strunk & White (sorry - it's a long story) for this implausible clause I found: "My first visit to Boston will always be rememembered by me."
On the other hand, if it said "My first visit to Boston will always be remembered by the Mayor, the City Council, and the Boston Police Department", it would be much more difficult to rewrite it into the active voice without, at the very least, radically altering the emphasis of the sentence, possibly greatly disserving the narrative which the reader is led to expect from such a beginning.
Which goes to show that the passive voice (even in the strict grammatical sense) has its uses, and teachers ought to adopt a more nuanced approach to it. Even if a given student is using the passive voice badly, telling them not to use it at all doesn't address the real problem and potentially deprives them of a useful tool for some situations.
'He was exhausted by the journey' is clearly in the passive voice; 'He was exhausted', on the other hand, is the verb 'to be' with the adjective 'exhausted' as complement.
[In the vast majority of cases it will work very nicely indeed. There may be a few cases of actives where accidentally an irrelevant by-phrase can be added, and perhaps the occasional passive that doesn't sound right when a by-phrase is explicitly added (if I was born in Oregon is a passive, it is one that does not allow an agent by-phrase); but those will be highly unusual cases. In general, John's test is very useful. And it completely negates the idea that passive clauses are vague about agency, of course, since the whole point of the by-phrase that you can usually add is to lay emphasis on the identity of the agent. —GKP]
[I'm sorry to say it is much more complicated than this. What Karen has said would not cover the passive clauses (underlined) in sentences like She went out and got herself arrested by the vice squad or I'll have your bags brought up to your room by the porter or This change went unnoticed by most people, where there is no auxiliary be; and it would not cover "concealed passives" like This thing really needs repairing by a professional, where there is no past participle.
Defining the class of passive clauses in English is not that simple, and I never suggested that it was. I'm only drawing attention to the fact that people keep using the term when they have not idea at all how it is defined or employed in the field of grammar but they throw the term around anyway. —GKP]