Reference : identifying seventh chords
There are several ways to identify seventh chords.
1. Identify the triad and the seventh interval that make up the chord:
See: identifying triads
2. By determining the key and the degree on which the chord is built:
In this case, it is a dominant seventh chord because the key is D major and the chord is built on the fifth degree. In major keys, a dominant seventh chord is built on the fifth degree.
