Calmly sends you a portfolio, cases, reports from previous clients, etc

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For a professional, these actions are a long-established practice and he will not refer to the confidentiality of information, etc. (unless he signed an NDA).

Builds a strategy based on your business objectives, not his own desires, ideas, or market trends. If you want to promote a specific brand, not the entire company, then he builds a strategy based on this data. A professional can recommend you what will work best and warn you, for example, that in your particular case Google is ineffective, but not nip this idea in the bud if you think you can find clients there.

Offers to try other tools. This is more relevant for agencies or teams that have in their arsenal of skills not only context, but also other digital tools.

And most importantly, he is honest with you. If it is vietnam mobile database impossible to get the number of clients you want with the help of contextual advertising and your budget, he will tell you about it directly and calculate a more realistic picture, rather than draining the budget, getting interest from it and feeding you empty promises.
Don't be enchanted by loud promises - usually they don't lead to anything good. Look at the adequacy of the specialist and the approach to advertising in general. Moderate forecasts, reliance on numbers and statistics, questions about your business tasks, website, experience working with Yandex and Google algorithms - you can try to work with this specialist or agency, even if your area is not in the portfolio.