What's the best technique to identify the guitar chords of a song?
I know the basic chords and also how to get into the rhythm of a song! But I'll always be spending huge amount of time to identify the chords :(
You can try using chordify. It's not always correct, but in many cases it's a pretty good start.
At a beginner level, the first step is to identify the root note of the chord - this is often (especially in most pop or rock music) the lowest note of the chord, so you should be able to hear and compare with a plucked note on your guitar.
If you can accompany a song just using the root notes then you will have an idea of the chords and the progression through the song.
Identifying whether it is an A minor, A 7th, A sus 4 etc comes more from practice and experience. You learn to hear what each type of chord sounds like and will be able to play along with a version of the chord.
Once you can do that, you are likely to find that although the chords may be right, you may be playing a different fingering than is used on the track. There are many ways to play each chord, not just because you can play in various places on the fretboard, but also because you don't need to have the notes in order, or even play all the notes in a chord, so to accurately play in the style of a particular artist, you need to learn their techniques - they may go for basic powerchords, or like to stretch across a wide range of frets etc.
To find the chords for a song (you mention guitar chords, which will be the same chords, but may have different voicings), first you have to establish what key the song is in. This you do by listening carefully to 1. What the first chord is. 2. What the last chord is. 3. A place in the song where it sounds like it could finish, even if it doesn't,or places where the same 'home chord' comes, often.This chord is most likely to be the key chord.
When you have this, call it chord one. The other two main chords in the song will be four and five. E.g. in G, G being one, C is four and D five.
Assuming it's in a major key, there may be minor chords in the song - six, two and three, often in that order of probability. Most pop songs will have less than all 6, in them. Many songs have only 3 or 4, and some (Achy Breaky Heart, Dance the Night Away) manage with only 2, these being one and five.
As Dr Mayhem suggests, to find a chord, listen to its basic note, and move one fret at a time up the bottom (fat) string of your guitar, until you come to the note that matches that chord. This stands a good chance of being the key chord. Put the radio on, and try this with every song that gets played, you'll be surprised how quickly you improve.