I recently took an online improvisation class with vibraphonist Gary Burton. In talking about scale selection he naturally starts with selecting the scale based on the chord in use and in cases where multiple scales would work look at the preceding chord for clues to the right scale. I can do that well enough if I have time to sit down and analyze the song ahead of time. But if an unfamiliar piece is in front of me I have a hard enough time just saying "okay G7, C major works" let alone thinking of all the other scales that work with G7 and relating them back to the previous chord and what I played over that. Maybe that will come with time and practice but right now it's a tough go. So I was wondering does looking at the key signature and the accidentals in the written music make reasonable markers for figuring out scales that work - maybe not necessarily the best scales to use but ones that will at least get the job done? For example if I'm playing a piece that's in G major but in bar 12 there's a C# in the melody can I reasonably use G Major and it's modes for the first 11 bars and then shift to D Major for bar 12?