Hi mbl,
Finding a suitable singing key for a specific song is a combination of trial and error.
Start off by determining the melody's highest and lowest note when played in the key you are most familiar with the tune in (say the key of C for example purposes only).
Play the song's melody briefly, having the singer try singing the highest/lowest melodic notes .The highest note usually should sit in the vicinity of the highest note range the singer can "comfortably" sing and the lowest note should sit toward the lower range comfortable for him/her to sing. If the highest note is a little too high for the singer to comfortably reach, then lower the key to B and try the test over again. If the highest note is still too hard for the singer to reach lower the key to Bb.
With electronic arranger keyboards it's much easier because you can continue to play in C yet transpose the key down by half-steps (via a touch of the transpose button) until you reach the best key for the singer. I hope all those years of piano lessons having to practice and memorize songs in ALL twelve keys hasn't been all in vain.
- Scott