Listen to how the artist performs naturally. It's best to get a grasp on how they handle a melody. Not all vocalists share the same range, nor do they sound best within that range. The key is finding where they feel most comfortable and sound the best.
Some artists sound great singing in a specific key. This is not uncommon. Many singers have listened and emulated music that has inspired them. They later develop a proclivity for making music their passion and livelihood.
You may have to alter or transpose your instrument or accommodate the key that fits them. I've known singers who sing in Em extremely well while others sound best in GM. It's all relative to the performer.
Determine where they sound best within the key. Register is very important to capturing a great vocal. If they are a tenor vocalist you will want to arrange the music to allow the singer to stand out.
Obviously, if you have a vocalist that sings baritone you'll transpose the music up to accent the higher register information while the singer handles the lower end. If there is a clash amongst frequencies, then there will be a constant struggle to make the vocal pop.
Define the range in which the vocalist will sing. If the key is too high and the respective melody the same, then the singer may not be able to reach high enough. They will strain their voice and lose control of their vocal. You have to avoid pitting a singer in that situation, because it will be detrimental to the song.
The opposite is true for lower-registered singing. One way to fix this problem is to lower or raise the key by a whole or half step depending upon what is challenging the singer.
This is very easy to adapt on the piano. On the guitar you can use a capo to raise the pitch of the instrument as a whole. If you need to lower it just tune it down.
Once you've established a rapport with the vocalist it will become much easier to compose music for them, so work out the preliminaries in advance.