It is actually not that difficult, if you know the root 6 and root 5 bar chords then you already know the notes on 3 strings, low and high E and the A string, leaves only 3 more to go; as for the D string, all the notes on that string are a whole tone below the E string so if the 8th fret on the E string is a C then the 8th fret on the D string is a Bb ( little extra here for this string; if you learn the D chord style al along the fretboard then the root is on that D string and you will know these notes already to ), then there is the G string and the B string; easy here to because for the G string the notes are a whole step below the A string and for the B string a whole step above that same A string,

So you see if you can see some system in it then it becomes easy, without a system it will take you a while.

Herman