you need to see the piece's sheet music, and count up the sharps and the flats noted beside the clef.

my pianist friend gave the following mnemonic devices for sharps Father Charles Goes Down And Ends Battle, and Battle Ends And Down Goes Charles' Father for flats. i may have reversed the two.

the key of C has no sharps or flats. then there is some other rule which by now i have completely forgotten which uses the above mnemonic device.

anyways, this is just to let you know that there is a very precise way of determining the key a piece is in. music is very mathematical and formulaic from a theoretical perspective, and there is a defined answer for almost everything in music, including how to determine what key a piece is in.

ask someone with experience in theory and he or she will be able to help you understand the rules for finding out the key.

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ok, i'm not really the eggman