though flight has tons of chromatic inflections, it very much has a key....you could look up sheet music for that to find the key...i'm sure any store that has sheet music will have it. it was originally composed for the violin which is a C instrument and so is guitar so you wouldn't have to worry about transposing it or anything...maybe changing the octaves....i'm not sure. it might also be written in a different clef like alto or soprano clef instead of treble.....but i doubt that, violin normally stays with treble.

for finding keys, you have to basically just use logic and the theory you know to figure it out. there's this workbook series called master theory...it's a good starting point to familiarize yourself with the keys and all.