It's a practice thing. Play lots of chords and listen to the sound.
Going with the bass notes is a good idea but you have to realize that he might not actually be playing the root note of the chord. It is still a good method.
What I tend to do is first find out what the key of the song is. If you play the melody, you can usually identify a single scale (sometimes not) that the song is in. Once you've identified the major scale the song stems from, you can use a chord corresponding to each note and the song usually uses one of these chords. Thus, you narrow down your choices. eg- If you identify that the song is in the key of C major-
The notes for the C major are
C D E F G A B C
So the chords are C, Dm, Em, F, G7 (or Gmajor), Am, Bm7b5 (rarely used)
The sus2/4 can be used for any of these but especially look out for it before a dominant 7th.
Note that another common chord used is the major of the 3rd instead of the minor (in this case, E) which would make the scale A harmonic minor though the G# note is usually used on the E and not outside it in most songs I know(not saying that it cannot).
As for how to make out if they're playing an augmented or a sus or any other, just practice the chords a lot and get familiar with the sound and you'll be able to tell. Practice is the only way I know.
And surely you’ll buy your pint cup !
and surely I’ll buy mine !
And we'll take a cup o’ kindness yet,
for auld lang syne.