The best way to learn key signatures, is to just memorize them. But, until you do, there are techniques that can help. If the key signature of a tune shows 3 sharps, it is in A. But, just knowing that is not enough. You need to know that the key signature represents notes that are played as a sharp. And, you need to be able to recognize these notes as sharps, even though they will not be indicated with a sharp next to the note. The key signature has already defined this for you.
In the example of A, the notes of the scale are:
A B C# D E F# G# A
So, when you see a C, F, or G, play the note one fret higher.
The Key Signature Flash Cards will help you get familiar with recognizing key signatures, and identifying which notes are sharp (or flat). As you go through the cards, name each note as you play it.
If you are having difficulty with this exercise, or naming the notes, go to the Mandolin Flash Cards and work on your reading skill. When you feel confident with recognizing notes, return to the key signature exercises.
Start Key Signature Flash Cards Slideshow