If you mean while you're looking at a song, usually the song starts and ends with the note of the key it's in.
If you mean how do you tell the key name, you look at the second to last flat and that is the name of the key. This works with all except F major (one flat, Bb) and only with majors.
Eb major. 3 flats (Bb, >>Eb<<, Ab)
Db major. 5 flats (Bb, Eb, Ab, >>Db<<, Gb)
F major, B-flat major.
In the order of flats in key signatures, even if the melody skips the B-flat, the key would contain it, because there is no such key with only an E-flat.
When reading flat key signatures, it is the second to last flat: The order of the flats go as such: B flat, E flat, A Flat, D flat, G flat, C flat, F flat. In this case that would make A being the second to last flat, thus the key would be A-Flat Major or f Minor.
The name of the scale is whatever the second to the last flat is. For example, if you have 3 flats, B flat E flat and A flat, the name of the scale is E flat, because it is the second to the last flat.
Key signatures are the sharps or flats at the beginning of the staff. To identify the key signature of a scale that consists of all sharps, look at the last sharp in the key signature. Whichever note the last sharp lies on, the key of the scale is one note above it. To identify the key signature of a scale that consists of all flats, look at the note directly before the last flat in the key signature. The second-to-last note is the name of the key signature of flat keys. However, you cannot use this helpful trick with the F Major Scale which only has one flat (B flat).
nobody answer
A flat major and its relative minor are key signatures with four flats.
F major, B-flat major.
In the order of flats in key signatures, even if the melody skips the B-flat, the key would contain it, because there is no such key with only an E-flat.
When reading flat key signatures, it is the second to last flat: The order of the flats go as such: B flat, E flat, A Flat, D flat, G flat, C flat, F flat. In this case that would make A being the second to last flat, thus the key would be A-Flat Major or f Minor.
The key signature indicates which notes are sharp, natural, or flat. Every key has a specific signature.
The name of the scale is whatever the second to the last flat is. For example, if you have 3 flats, B flat E flat and A flat, the name of the scale is E flat, because it is the second to the last flat.
Key signatures are the sharps or flats at the beginning of the staff. To identify the key signature of a scale that consists of all sharps, look at the last sharp in the key signature. Whichever note the last sharp lies on, the key of the scale is one note above it. To identify the key signature of a scale that consists of all flats, look at the note directly before the last flat in the key signature. The second-to-last note is the name of the key signature of flat keys. However, you cannot use this helpful trick with the F Major Scale which only has one flat (B flat).
There are three keys which are called "enharmonic keys", the three enharmonically equivalent key signatures are B major/C-flat major, F-sharp major/G-flat major, and C-sharp major/D-flat major and likewise their relative minors. B/C-flat, F-sharp/G-flat and C-sharp/D-flat all share the same pitch but are just notated in two different ways. In an other way the keys mentioned above are just one key going by two different names but they use different accidentals and are written on different lines/spaces.
It shows which notes in a piece are natural, sharp, or flat. Without a key signature, every single accidental would have to be written in instead.
A key signature tells a musician what key they are playing in and how many sharps or flats they must use. For example, a key signature with a B-flat would mean that the musician would play a B-flat throughout the piece unless indicated otherwise.
If you are talking about key signatures Bb major has 2bs but Bb minor has 5bs.