FOR SHARP KEYS ONLY:
The name of the key is a half step ABOVE the last sharp in the key.
Using A as an example, it has 3 sharps (F#, C#, G#). The last sharp is G#. A half step from there is A.
The tune is in A.
FOR FLAT KEYS ONLY:
The name of the key is the next to LAST flat in the key.
Using Eb as an example, it has 3 flats (Bb, Eb, Ab). The next to last flat is Eb.
The tune is in Eb.
An exception to flats is that the key of F only has one flat (Bb). So, there is no next to last flat.
The key of C has no sharps or flats.