Chords can be detected by their unique sound, and by the associated feeling they evoke.
Associating such a feel to each chord is subjective and can be quite abstract, so the focus should be on how tense or resolved the chord sounds.
Here are few examples to illustrate this:
A Major chord is considered a 'resolved', or a happy chord:
A diminished chord, is considered to have a distinctive tense sound, or a 'dark' timbre: