Choosing a marketing agency is hard for a simple reason: you are buying something you cannot inspect in advance, from people whose job is persuasion. The good news is that real capability leaves evidence, and you can learn to read it in a week of looking.
Start with their own presence. An agency that promises you leads should be visibly good at generating their own: a fast website that explains clearly what they do, work they can show, reviews from named businesses, content with an actual point of view. If their own marketing is generic, yours will be too.
Look at proof, not portfolios. Pretty creative matters, but ask what happened after it shipped. Good agencies talk in outcomes: enquiries, cost per lead, revenue, retention. Vague answers about impressions and engagement are a signal that numbers were never the goal.
Ask how they will spend the first thirty days. The right answer involves understanding your business, your margins, and your customers before spending your money. The wrong answer is a package with a price that was decided before they met you. Marketing that ignores your economics is just expensive noise.
Finally, judge the honesty. A serious agency will tell you what will not work, what they do not do, and when marketing is not your real problem. That candour early is the best predictor of how they behave when a campaign underperforms, because eventually one will, and you want partners who tell the truth on a bad month.
Take your time, talk to two or three, and pick the one that asked the best questions. The agency that tried hardest to understand your business is usually the one that will grow it.