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check his background in the feild. His response to job and knowledge of market.
First, his experience and knowledge of the field of work second responsibility for the work
The appropriate questions that we know from his answer to his way of thinking and the limits of his responsibility
Just select the person who has the persona that you think it is fit for your product under promotion. Just make sure to select more than one to make sure that you are not paying to much money. Also, make sure that this person doesn't represent products in different industries because in this case he would have lost big part of his impact.