I am a singer who sings by ear. I've had some training, but not much in the way of theory. My question to you is, how do I tell what key I sing a song in. I am about to join a jazz trio and will have to know this. I do have a piano at home that I play by ear. Please help! Thank you!

You can tell the key a song is written in by looking at the sharps or flats at the beginning of each line of music. These sites explain.

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http://cnx.org/content/m10881/latest/

It helps if you learn the various major scales then when there's a flat in the song, you can generally hear it and assume that it may be in a different key? Listening by pitch I suppose. Hope this sort of helps...