Both, actually, but the sheet music was the principal source. Which is, among other things - why I tender to refer to key signatures rather than keys!

When I was playing with the Jubilee Jazz Band in the '80s, there was a band habit of responding to the question, "Key?" by a display of fingers - downward-pointing fingers meant flats and upward-pointing fingers meant sharps. So - 3 fingers down = Eb, and 2 fingers up = ...D!

This was used for both major keys and relative minors, so if I'd asked what key "Petite Fleur" was in, I'd have been shown 3 downward fingers... Bb OR Gm. :-)