| Slaine,
This is an easy one... obviously, you have atleast some elementary grasp of scales. You know that 'D' is a half-step below 'Eb', the 8th degree / octave of the scale, and the scale ends with a half-step to the octave. Therefore, the 'D' - a half-step below 'Eb' - is the seventh degree of that scale.
If however, say someone asked; what degree is 'Ab' in a 'Bb' scale? Utilizing the reverse circle of 5ths, or, as I like to teach, the circle of 4ths, you would quickly know there is no 'Ab' in a 'Bb' major scale. It would actually be part of minor scale...
Typically, at the beginning of a piece of music - just after the bass clef - the musical key will be indicated by how many, if any, flats or sharps.
Your first sample 'C' major, has no sharps or flats. Sticking with your 'Eb' second example, and following the 'circle of 4ths', one flat would indicate a key of 'F'. An 'F' major scale's "4th degree" is 'Bb'. And, ofcourse 'F' is the "4th" of 'C'.
Continuing in 4ths;
The "4th degree" of 'F' is 'Bb'. Its "4th degree" is 'Eb'. So, a key signature with two flats, would be 'Bb'. The flats for that key are 'Bb' and 'Eb'.
The "4th degree" of 'Bb' is 'Eb'. And again, the 4th of 'Eb', is 'Ab'. Three flats, 'Bb', 'Eb', and 'Ab'... the key signature?? 'Eb'
If you follow along, you'll see the 'circle of 4ths' continues; Bb, Eb, Ab, Db, Gb, Cb, and Fb. (all seven degrees from top to bottom)
The aforementioned 'circle of 5ths' covers the sharps in the same manner as the circle of 4ths is on the flats...
The 5th of 'C' is 'G'. One sharp, F#. 5th of 'G'?... 'D' Two sharps?? F# and C#
-continue following the pattern of 5ths... both of these methods are quick references and part of knowing music theory.
Learn them and, the next time you're hit with a 'musical interval' question, you'll know the answer right away.
Hope this doesn't make any more confusing than it may already seem... michael | 
Trouble with Intervals 
