How well do they understand your business?

How varied is their portfolio?

How clearly do they communicate?
Ambiguity and gaps in understanding lead to most break-ups between brands and creative agencies. Look out for agencies that ask you the right set of questions and come back with precise information on the process and other project parameters. While clear terms on the cost and duration of the project are crucial, look out for companies that also talk about a benchmark quality and success metric in the context of your potential engagement with them. Promptness in email responses also reflects the professionalism and commitment of the agency to serve you.
These are some key aspects that can be evaluated even before you commit yourself to your first engagement with a creative agency.
Feel free to comment below with your thoughts. Do mention the challenges you have faced in working with creative agencies in the past.