This is the best rule I've ever seen for identifying and eliminating passive voice from your writing:
Insert the phrase "by zombies" after each verb. If the sentence still makes sense, it's most likely an example of passive voice. Fix it.
1) The murder victim was shot (by zombies) four times.
2) I shot (by zombies) the murder victim four times.
The first sentence still makes sense, which means it's in passive voice. The second example is in active voice, so it doesn't make sense.
Passive voice is the written equivalent of up-talking. We're not schoolchildren, nor are we zombies. We are writers.
This idea was brazenly stolen (by zombies!) from Rebecca Johnson.