Look at the key signature on the piano music. A G Clef is at the very left, and then there is a series of sharps or flats before the time signature, or there may be no sharps or flats at all. The only major key that has no sharps or flats is C, which is the root of that scale in that piece of music.
See if the key signature has a series of flats. If so, look at the second to the farthest flat to the right on the staff's signature. That is the root note of the key. For example, if there are five flats in the key signature, look at the second to the last one to the right. It is on the second to the top line of the staff, which represents the D note. Since D is flat, according to the key signature, the root note is D flat. If the signature only has one flat, which is B flat, then the key is F.
Look at the key signature to see if it has sharps. If so, look at the sharp that is at the farthest right in the signature. Go up one line or space from it, and you determine the root note of that key. For example, if there are two sharps, the farthest one to the right is in the space for the C note. Go up a half step from C Sharp, since the signature indicates that a C sharp is used instead of a C, and determine that the key is D.
Listen to the music to determine if there is a possibility that it may be written in a minor key. Minor keys flat the third, but you may be able to tell by listening. Major keys sound happy and straightforward, and minor keys sound sadder and more ethereal. Each key signature for a major scale also has one for the minor, which is called the relative minor. Count six whole notes up from the major root to find the relative minor. For example, if the key signature has one sharp, which is the F sharp, then the key signature is for G major. Count six whole notes from G up to E, which is the relative minor root of the piece.