There are several aspects to look for:
- Track record: How long have they been in business, how many partners, customers, employees do they have? Are they available, can you have a face-to-face meeting with them if necessary?
- Team size: How big is the company's development team? How many mobile developers are there? What is the size of the consultancy and design team?
- Technical knowledge: How long have they been in the mobile development business? Can they offer several different development options? Do they have a sufficient number of experienced and seasoned developers?
- Proper process: Do they have proper processes in place, can they explain in a logical way what their development process is?
- References: Do they have references that are of sufficient quality, that can be easily checked and that work well?
- Availability: Are they available for your project, can they provide a development team?
- Good match: When you talk to their colleagues, do you have the feeling that you could work well together? Do they offer the kind of service that you really need?
If someone ticks all the boxes, work with them.