Both are a tritone appart, both sound the same. How can we differentiate when an interval is an augmented fourth or a diminished fifth?
In voice leading we learn that when the spelling of the tendency tones (7th and 4th degrees) forms a diminished fifth they are going to resolve inwards (towards each other), and when the spelling of the tendency tones forms an augmented fourth they will tend to resolve outwards (a way from each other).
This means that in V7 resolving in I in C major (G7 to C) the tendency tones are B and F. Depending on the voicing B and F can be either an augmented fourth or a diminished fifth.
What's the difference?