The only real time you need to sync your audio with your footage is if you take a separate recording of the audio and the footage or if your recording is out of sync with the audio that was captured with the recording device.
The real trick to syncing audio and footage is to use a video editing software - like iMovie HD - and separate the two if they are not already separate.
Once the footage and audio are separate, set them in the editor so that they are fairly close. It is usually best to move the audio to sync with the video.
As you get closer to syncing your audio with your video you will notice the more pronounced syllables start to match with the sound you hear.
*Also note that this is easier with headphones.*
Once you start to notice these matching syllables it will become clear when the audio and video are a perfect or near perfect match as the voice(s) will no longer seem dubbed.