Today 3D character designers are in high demand, so there should be a structured and efficient way to hire professionals for this particular type of 3D modeling.
Firstly, consider the professional background. 3D modelers are working on a different basis: some at the studios, some are freelancers. With the first type, you are provided with more guarantees that the work will be delivered on a high level. When working with freelancers, make sure they are highly ranked and have close to 100% positive feedback.
Secondly, ask about the experience. 3D character artist's experience must be relevant to the project you are going to recruit them for. The more solid experience, the better, however, do not rely on this indicator the most. Throwing a glance at the portfolio will shed more light.
Thirdly, look through a portfolio. Every respected 3D artist will be ready to demonstrate their polished portfolio. The artworks have to display the level of skills, genres of artworks, placement of characters, etc.
A hint: find a character related to your request and see how the work was rendered. Remember that a good 3D character designer must ace such tools as Photoshop, Substance Painter, 3D/Maya, and desirably ZBrush. Let them be supplied with 3D art examples. Put creativity to the highlight.
When recruiting a 3D artist, don't hesitate to ask about their opinion on characters they have created or challenge them into the task of creating a new one. Do you like their ideas? Are they creative and coincide with yours? Take these points into consideration and you'll surely get a great expert for your project.