Here are some of my memory pegs.
Sharps. See God Destroy All Earth By Fiery Chaos. No sharps = the C scale. One sharp = God or the G scale, etc, etc.
Which sharp? Fat Cats Go Down Alleys Eating Birds. The G scale will have one sharp and I know it will be the F# (from running the memory peg Fat Cats Go Down). The D scale will keep the F# and add the C#, etc, etc.
Flats. Farmer Brown Eats Apple Dumplings Greasily Cooked. Same thing - Farmer or the F scale has one flat the Bb. Brown or the Bb scale has two flats the Bd and the Eb, etc, etc.
Key signature has 4 sharps - which key is that? God, Destroy All Earth - so four sharps in the key signature tells me it is the E scale and the E scale will have (Fat Cats Go Down) the F#, C#, G# and the D#.
Now up pops the devil --- Lets say the key signature tells me it is the C scale. This also tells me it could be the Am scale. The key signature tells me the major scale and the major scale's relative minor scale. Each have the same notes. Chords are the same, but, in a different order. If C the principle chords will be the C, F & G7 chords, if Am the principle chords will be the Am, Dm & Em. The chords give it a major or minor sound.
And yes to having a Big Pink Eraser handy.
The big pink will not smudge.
Have fun.