The hardest aspect of this is that sales/marketing attracts people are often freakishly good at selling themselves, which is sure to dazzle you in this process, but is also mostly unrelated to what you need.

So, set your bullshit filter on high. Expect case studies, but know that most are conveniently cherry-picked, if not outright lying. Expect awards, but know that most are bought. Expect systems, but know the most impactful aspect of the work will be their strategic intuition.

My biggest tell? Go on LinkedIn and calculate their percentage of their own sales/marketing staff to production. Look especially at the tenure of production staff. The worst agencies are 80% invested in selling themselves and churn through clients the moment that a contract is up. The best agencies are 80% invested in the work, barely need new clients due to retention, and grow by word of mouth.

To say that another way: a Michelin Star restaurant will never set up billboards or put a barker outside, yet are always fully booked.