Don't assume that your client's needs can only be met by a large (global) consulting firm. This assumption is unfounded.
You should be fully aware of the differences and levels of experience, knowledge, and expertise between different technical specialties and disciplines within a given company.
Be familiar with typical project/site situations and past experience when hiring project engineering services.
Determine an engineering office only if the scope of services required is clear
Avoid choosing an engineering consulting firm that struggles to work in-house as a team, coordinating input data, and integrating input data to support project development.
Choose a technical consulting firm that fully understands the laws, regulations, permits and environmental standards required to design and build an efficient project.
Start the call immediately. Ask questions and learn more about how the company works.
A list of authoritative references from consultants should be available and those references should be provided.