It totally depends on the song, plus the voice range of the singer. There is no easy way to "know", but it does get a little easier after several years experience.
Once I know what key a singer does one song, I can relate that to the key in which I do the song, and can transfer that knowledge to any more songs the singer might do. As a start, ask them to sing a few notes of the song without music. Then raise it a key or two and try it there. If you find the key isn't correct, there's not much to do but either modulate or stop and start over. This is best handled by joking with the audience about it, such as blaming the drummer for starting in the wrong key.
As far as determining your own key for a particular song, you will just have to practice, trying various keys until you find the one most comfortable.
As a rule, you can do ballads and soft songs a little lower, but high-energy songs sometimes call for a little "reaching" in order to get the right feel.
I know this doesn't help much, but until you determine your own keys for each song, I wouldn't even attempt to play for a guest!
DonM
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DonM