2.17How to recognize a sharp key quickly

There's a useful shortcut.

For sharp major keys:

The final sharp in the key signature is one semitone below the tonic.

Example:

F♯ C♯ G♯

Last sharp = G♯

One semitone above = A

Therefore:

A major

Another:

F♯ C♯ G♯ D♯

Last sharp = D♯

One semitone above = E

So:

E major

Useful when reading sheet music.