I'm working my way through the Open Music Theory digital textbook (http://openmusictheory.com/).
There is a chapter that describes intervals and how to determine their quality. The introductory method given is a table with Chromatic Intervals along the side and Generic Intervals along the top. The table is depicted below:
Using the table, we can see that C-G is a P5 because of the five diatonic steps from C to G (C, D, E, F G) and the seven semitones (i7) from C to G (C#, D, D#, E, F, F#, G).
The author goes on to recommend a more intuitive method for memorizing intervals called the 'white-key method' (http://openmusictheory.com/whiteKeyMethod.html).
Using the logic of the white-key method, all fourth intervals are perfect with the exception of F-B which is augmented. By inverting the intervals and by extension of this method, are all fifth intervals therefore augmented with the exception of B-F, which is perfect?**
**I know there is a flaw in my logic because looking at C-G, I come to the correct solution using the table (P5) but using my understanding of the white key method, I see it as an A5.