The differences between the quality of seventh chords has greatly to do with the 3rd and the 7th of the chord. Listening clearly for those and remembering your intervals, for example, if it's a minor third the chord can't be major. Practicing listening to each note while playing the chord harmonically can help develop the ear to distinguish the differences. In a major 7th chord, you have a major triad ( C, E, G) with the seventh scale step of the C major scale (which is B). That B is important because it is the leading tone to C. Once you hear it you want it to resolve up to C, this is when listening to the 7th of the chord comes in handy. In a minor 7th chord, we have ( C, Eb, G, Bb). The minor 3rd and the minor 7th are what makes this chord different than the major, however, the dominant chord ( C, E,G, Bb) can sound very similar to the minor chord because of that minor 7th, so make sure you can identify that 3rd. Then we have our last two seventh chords which are the half-diminished and the fully diminished. The half-diminished chord (C, Eb, Gb, Bb) sounds more mysterious than the diminished chord due to the minor 7th instead of a diminished 7th. In a fully diminished chord you have (C, Eb, Gb, Bbb) that Bbb is what will make the sounds of the chord much more darker like something you'd hear in a scary movie. Another way to think about the fully diminished chord is listening for a major 6th or if you're in an inversion, for a minor 3rd. Let me know if you would like me to clarify anything!