I know that when a key signature has sharps, the key indicated is always one half step above the last sharp in the key signature:
For example, the key of D Major has two sharps - F# and C# - D is one half step higher than the last sharp.
Can anyone give me a similar easy rule that works with flat key signatures?
You can find the second to last flat, and that's the major key signature. Flats are easy!
For example:
The second to last flat is on G, so Gb Major.
This trick works for all of them except, of course, F Major, which has one flat:
Just remember that one flat is F Major.
For minor key signatures, you can count up six scale degrees on the major key signature, or count down three half steps. I usually count down, because for me that's easier, but it's technically more correct to view the minor key signature as starting on the sixth scale degree.
(Taken from another answer of mine from this question: Finding The Key Of A Song, which I initially misunderstood and this answer isn't relevant to it, but I am glad I left it up!)
The last flat shown is the fourth note from the scale of the key. Thus just Bb shown, this is the fourth note from F, the key in question.
A key sig.can also mean minor key, so with last (only) flat as Bb, count up two tones to the key note, of D. Thus, Dm.
