If you’ve not spent much time ear training here are a few suggestions to get you started:
Note Against Chord
Play a C major chord and then proceed to sing the notes of the C major Scale, start with the root, when you sing the note make sure the chord is audible since you want to hear how the note sounds against the chord, once you’ve sung the note, strum the chord again and try to imagine the note in your head, continue this with every note of the scale. Once you have finished, change the quality of the chord and the subsequent scale and follow the same procedure. If you can’t sing, record the chord or program chords into music software of your choice, and play the notes of the scale against the recording; the objective is to enable you to recognise any note played against any chord.
Identifying intervals is another good practice, I’m hoping that the colour coded circles on the fretboard in the other pages help you to understand how intervals look on the guitar fretboard, this should aid you in playing instantaneously the intervals you hear in whatever context.
Popular Culture Association
A good way to practise intervallic identification without the aid of an instrument is to identify the interval with popular musical themes; they can be derived from movie soundtracks, national anthems, songs that you can instantly recognise, it can be anything that works for you. Take the first two notes of the melody to the Star Wars soundtrack; what you have is an ascending interval of a perfect 5th, this melody is probably instantly recognisable by many of you reading this, so already you know what a perfect 5th sounds like without an instrument. Another example could be an ascending perfect 4th interval as exemplified by Mendelsshon´s Wedding March, this is the music played on church organs at weddings.
The idea of intervallic identification without an instrument is to associate familiar popular culturalisms that are already stored in your subconscious and relate them to new information, in this case, the ability to identify musical intervals. In addition, I can´t over-estimate the importance of transcribing as this is not only good for your technique and sight reading, but very useful for intervallic identification.
May I suggest Jamey Aebersold´s Jazz Ear Training course book, an excellent resource for helping you to develop better listening skills.