I think it's pretty much trial and error to begin with. Then, you'll begin to notice which keys appear to be the most comfortable for you.
That said, depending on the songs different keys may come into play, especially if the song goes very low (for you) or very high (for you).
this matter of KEY can be a sensitive one for song circles/groups of singer-players. those who create the song sheets for song circles need to favor, I believe, those who will be leading the group. If it's uncomfortable for the group, well... that's how it is. If it's uncomfortable for the leaders, well... you can see how that could affect the entire group.
Our group at SUPA are very accommodating and polite as we don't usually get complaints about songs being too high or too low. Of course not everyone sings! But the keys are tailored to the moderators so at least those who are leading can sing strongly for the group
It is a matter of concern, however.
Back to the question at hand: Trial and Error, man, no getting around that.
For me, with a lower register, I generally down-tune my performance uke (for solo work) about 1/2 to a whole step.
Again that's when I'm playing and singing alone. With a group, well, it's back to GCEA and a little voice straining
keep uke'in',