First and foremost, do some research online. Check out the websites of different custom mobile apps development companies in Illinois and look at their portfolio to see what kind of work they've done in the past. Make sure to read reviews from previous clients to get an idea of how satisfied they were with the work. Once you have a shortlist of potential companies, it's important to schedule a meeting with them to discuss your project in detail. During the meeting, ask them about their experience, their process, and their pricing. It's important to find a company that not only has the technical skills but also understands your vision and can work with you to bring it to life.