Hello

I would have the "singer" sing his lowest and highest comfortable notes as a test. Once you know the range one can rather easily find what key to play in if one can look at the music and see what the highest and lowest notes are. If you are doing it by ear one should be able to at least approximate the highest and lowest.

Another quick test is to try in one key, lets say the key of C if it goes well, great, if not going up a fourth to F or to G (a fifth) will most often find a usable key quickly.

Scott Langholff