figure out the melody of the song.....most modern rock songs today don't follow the normal majory key and opts instead for a dominant seventh and a kind of bastardized key that's somewhere between lydian(V) and ionian (I).

best way to remember all the flat and sharps is to just work on it...i guess for an easy trick....in the sharps, the last sharp written is 7th note of the scale. for flats, the second to last flat is the name of the key.

take for example the key of Ab maj has 4 flats: Bb, Eb, Ab, and Db.

and for sharps...the key of F# major has 6 sharps: F#, C#, G#, D#, A#, and E#.

for the examples, i listed the flats and sharps in the order that they're listed when you write out a key signature.

i will say though.....if a song has 3 chords, one of the chords is inevitably the one chord.....that doesn't necessarily mean is the one in the ionian mode, but there's a one chord...i wrote a song once in the lydian mode that went around the chords F, G, and dmin. the root chord of the scale i wrote the song in was F.