I have been learning guitar by ear for the past few years. I was listening to a song I've been trying to learn for a while, and realized numerous notes I had thought were single notes were actually power chords. As a result, I would like to know how to distinguish power chords from single notes on songs which involve the guitar.
What you can do is to learn what they sound like on their own, and then practice them in context of the song. With practice, you will be able to isolate the guitar parts and figure out if they're power chords or not.
You can do this by playing the power chord parts alongside a drum track, or with minimal accompaniment from the other instruments so they don't make your own guitar sound hard to distinguish from them.
This website has drum tracks for different types of beats at different BPMs, as well as "jam tracks" for improvisation practice: http://www.guitarlessons.com/jam-tracks/
I suggest using a drum track close to the BPM of the song you're playing.
If you are not able to distinguish power chords from single notes yet, it also helps to look up the tabs or notation of the song, instead of, or in addition to learning the song by ear.