Good question. Depends on what instrument you are playing how you went about knowing what notes are in a chord -- IMHO.
a) From years of playing, they know off the top of their head it A C# E G# Kinda, yes
b) At one point they memorized all the popular chords using flash cards Memorized
c) They might not know the pitches, and to figure it out they count semi-tones from A You could.
d) They might not know the pitches, and to figure it out they think of the key and count scale degrees Could
e) It varies, everybody has their own way Yep, I think this hits it on the head.
f) They search google Or use a chord generator.
g) Some other way Yep, back to "e".
Rhythm guitar. All you really need to know is how to finger the pattern, to get the right notes. I let the pattern take care of having the correct notes.
So if I am using fake chord or lead sheet music the sheet music will give me the chord name - all I need to know is how to finger the pattern that gives that chord. However, when I play bass guitar I'm expected to play the notes of the chord one note at a time and we do not strum so our pattern is going to be different - however patterns still work, if I know what notes I will need.
On bass I see the Amaj7 chord and I say to self; OK it's major and has a major 7. So I'm looking for a root, a 3, 5 & 7. Now I'm always looking for a root, 3, 5, & 7 and if I know where the root is on my fretboard the 3 is always in the same spot relative to the root. The 5 and 7 are also in the same spot relative to the root - just waiting for me to use them. Now the notes I use in my bass line can be:
Major Scale Box.
G|---2---|-------|---3---|---4---| 1st string
Just roots, i.e. A-A-A-A to the beat of the song when I'm under the Amaj7 chord.
Or R-5-R-5 or R-5-8-5 or any combination of those three notes.
And if I really want (or need) the rest of the chord tones then I could throw in the major 3 and major 7 into the mix. http://www.youtube.com/watch?v=obFcsEtFIKA
It kinda depends on the instrument you are using and whether or not you rely upon The Pattern or need to be involved with individual notes, i.e. playing piano you will need to know what notes are in each chord or rely upon a "frozen hand" pattern to sound those notes. Google frozen hand, piano.
Here is a cheat sheet - you are always hunting for the correct root, 3, 5 & 7:
Major chords will have a 3. Minor chords will have a b3. Diminished chords will have a b3 and a b5, augmented chords will have a sharped 5. Then we get into the sevens. It's not hard to put that to memory.
Major Triad = R-3-5
Minor Triad = R-b3-5
Diminished Chord = R-b3-b5
Maj7 = R-3-5-7
Minor 7 = R-b3-5-b7
Dominant 7 = R-3-5-b7
Ĺ diminished = R-b3-b5-b7
Full diminished = R-b3-b5-bb7 Harmonic minor and melodic minor will use the full bb7
Major Pentatonic = R-2-3-5-6
Minor Pentatonic = R-b3-4-5-b7
Blues = R-b3-4-b5-5-b7
Major Scale = R-2-3-4-5-6-7
Natural Minor Scale = R-2-b3-4-5-b6-b7
Harmonic Minor Scale = R-2-b3-4-5-b6-7
Melodic Minor Scale = R-2-b3-4-5-6-7
Ionian same as the Major Scale.
Lydian use the major scale and sharp the 4 - yes, it’s that simple.
Mixolydian use the major scale and flat the 7.
Aeolian same as the Natural Minor scale.
Dorian use the Natural Minor scale and sharp the b6 back to a natural 6.
Phrygian use the Natural Minor scale and flat the 2.
Locrian use the Natural Minor scale and flat the 2 and the 5.
The root, five and eight are generic and fit most any chord. Remember the diminished has a flatted 5.
The 3 is generic to all major chords. See a major chord R-3-5-8 is a generic bass line that will work.
The b3 is generic to all minor chords. See a minor chord R-b3-5-8 is a generic bass line that will work.
The 7 is generic to all maj7 chords. R-3-5-7.
The b7 is generic to all dominant seventh and minor seventh chords. R-3-5-b7 or R-b3-5-b7.
The 6 is neutral and adds color, help yourself to 6’s. I like R-3-5-6 for major chords. Has a great sound.
The 2 and 4 make good passing notes. Don’t linger on them or stop on them, keep them passing.
In making your bass line help yourself to those notes, just use them correctly.
Remember roots, fives, eights and the correct 3 will play a lot of bass.
See if that turns the light on.