When you Google "content marketing agency," more than 153 million results appear. Finding and vetting the best content marketing agency for your needs can seem like a daunting task, but it's critical to your long-term success.

So, where do you start?

I. What to Look for in an Effective Content Marketing Agency

1. Long-Term Engagement Case Studies

Content marketing agencies are quick to brag about their achievements, but often these accolades (e.g., Best X of the Year) can be merely a facade. The best way to evaluate a company is to look for in-depth, long-term content marketing case studies, which demonstrate the following:

Conversely, the lack of these long-term case studies may prove these issues:

Note: Just because an agency doesn't have a case study in your particular vertical doesn't mean they are incapable of producing high-quality work for you. Content marketing skills are transferable across niches and broad verticals, and the best agencies can nurture publisher relationships as needed.

2. A Large and Diverse Portfolio of Work

In our survey of 500 top-tier publishers, we discovered that articles, infographics, mixed-media pieces, data visualizations, images, videos, and interactive maps were the most sought-after content formats.

While case studies are an excellent place to get an in-depth view of a content marketing agency's long-term results, a portfolio provides a quick snapshot of the breadth of the campaign types an agency can produce, the types of clients they work for, and the quality of their campaigns.

3.On-Site Client Testimonials and LinkedIn Recommendations

Client testimonials are one of the most authoritative forms of recommendation for a content marketing agency. These are people who have worked intimately with the agency's team, have no bias, and genuinely found value in their results. Getting a phone interview with these clients, if possible, can be an added bonus.

In addition to on-site testimonials, LinkedIn recommendations can provide more personal insight into the people who are producing your campaigns. Research the company founders to reveal their previous experience with successful acquisition and to gauge how much the leadership team interfaces directly with clients versus passing these duties to junior associates.

4. Industry Columns Demonstrating Thought Leadership

A great way to evaluate if a company in an authority in their industry is to research the published work of their team members. By reading the columns of their founders, you can gain more insight into the depth of their knowledge, how they view the industry, and the direction they see it headed.

When researching the founders on LinkedIn, see if any columns are listed under their "Publications" section. If you can't find anything there, research the founders' names in Buzzsumo by searching "author: [name]" to see if you can pull up any of their authored work.

Discern the value of their posts by examining the comments, the social engagement, and the recency to get a big-picture view on how they see the industry and whether their views are undeveloped and biased or advanced and thought-provoking.

5. Proprietary Industry Research Informing Processes

The best content marketing agencies stay ahead of the curve by continuously spearheading proprietary research, which is used to refine their content creation and influencer marketing processes. In an industry where innovation and saturation happen at a relatively fast pace, it's important to constantly evolve through continuous education and testing. A company that constantly improves its process also helps your brand stay ahead of competitors who are practicing the same tactics.

6. The Metrics Used to Evaluate Success

Many companies have been scarred by SEO scam artists, who preach their ability to secure "cheap paid links," "manual submission services," "guaranteed rankings," and more. Fair warning: You get what you pay for.

When evaluating the best content marketing agency for your needs, look at how their KPIs align with your goals. Based on our survey of more than 30 content marketing agencies, the top metrics clients use to measure content marketing success are through the number of the following benchmarks: leads; high-quality links; total social shares; purchases, downloads or subscriptions; and unique visitors.

In addition, the most successful agencies will often achieve increased organic rankings for their clients over a long-term investment in high-quality content.

7. Authoritative Speaking Engagements

Although an agency may claim to be strategic, you should look for external proof of strategic thinking, which becomes obvious in case studies, columns, and speaking engagements.

Similar to columns, a company's speaking engagements allow you to evaluate their insights and authority in the industry. Do they only talk about one topic, or do they cover a diverse range of topics in their industry? Do they always repeat the same tips, or does their strategy evolve as the years pass? How authoritative are the events they're invited to speak at?

8. A Comprehensive Strategy

While ordering piecemeal content (e.g., one infographic) may be a cheaper upfront investment, the benefits are often short term and lackluster.

The content marketing industry is largely dominated by retainer packages, which often include strategy consultation, ideation, research, production, design, and influencer marketing. These comprehensive strategies provide longer-term results and greater reach than piecemeal campaigns.

9. Who's Doing the Work?

Often during the sales process, a partner or senior executive at the agency sells you on their personal accolades and then passes you to a junior associate to execute the work. While this is a common practice, you can still ensure that the people producing your campaigns are knowledgeable and skilled.

Ask these important questions:

10. Boutique Agencies, Big Results

Often smaller agencies become so consumed in doing great work for their clients that they forget to invest in their own brand. Due to the "Cobbler's Children Syndrome," you should never overlook agencies that may not have dedicated as many resources to their own brand as they've dedicated to marketing their clients'-just because an agency has a smaller social following or lacks a blog, that doesn't preclude them from being an authority in the industry. In fact, often times these boutique, specialized agencies are nimble, allowing them to evolve quickly and pave the way with innovation. In comparison, older, bloated, and more traditional agencies may have difficulty keeping up with change.