Well, there are hundreds of guitar chords...
When a guitar player is listening to a song and trying to figure out the chord progression, how does he figure out what a chord is if its really obscure? Does he have to have heard the chord before, or can he tell by the sound that the chord is a major or minor, or diminished, or w/e and what key the chord is in?

Like lets say in a chord progression theres a Asus4. How does the guitarist figure out what chord it is? DO they have to have heard it before or is there a certain technique to decipher what chord it is?

Thanks!

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I make sure ive got the root note right, then just try and work it out by ear. experience with music helps

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This.

I can tell majors and minors fairly easily, but more often than not I end up sounding out the whole chord by intervals.

Sometimes I just happen to get it right the first time, once I've identified the root note seperately from the rest of the chord I just begin picking apart the other pieces.

I do this in my head though. Like if I hear a chord I can hear that it is a mix of several tones and not just one thing. In my head I'm going "do fa ri ma so" and then putting it all together to see if it sounds the same, but not necessarily like that I just try to hear the seperate tones as they sound from the guitar rather than actually saying "do fa ri ma so".