You can't ignore it: The internet has become a central part of the marketing mix for just about every business out there. As a result, demand has grown in recent years for online marketing agencies that can help companies navigate the increasingly complex world of search, social media, email marketing, affiliate marketing, and more. And in response, tons of agencies are popping up, promising you the world when it comes to helping generate online visibility and maximum return on investment. But how do you know if your agency is really delivering what's best for your brand?

It's all about transparency. Because online marketing is such a highly specialized expertise, it's too easy for an agency to "fake it" -- that is, over-promise, overcharge, and under-deliver when it comes to results. To really know where your online marketing ROI stands, look for these top five red flags when evaluating an agency -- whether you're working with one already, or looking to bring one on board.

Is your agency regularly testing different opportunities? Let's face it: Not every campaign works the first time. Or the second time. Or the third. Any online marketing agency worth its salt should tell you right off the bat that it will need to try a combination of different channels and approaches over time in order to achieve real, sustainable results for your business. In the world of online marketing, there is little value in one-and-done campaigns that drive initial traction but can't sustain attention over the long term.

That means it's critical to continuously analyze, fine-tune, and update your campaigns; it's the only way to truly optimize your spend, while maximizing your return. Be wary of an agency that is not pushing you to test and adapt its strategies on a regular basis, and share its results -- both good and bad.

Do you get regular, detailed reports? When it comes to online marketing, the numbers simply don't lie. Like any marketing effort, your online strategy is designed to yield some form of measurable, quantifiable results, such as qualified leads or web site volume. Even if a campaign isn't working -- and again, some just don't -- your agency should be able to tell you why, using specific numbers and metrics. It's a major cause for concern if an agency is not willing to give you full access to your activity and results at every level, through real-time reporting, testing tools, and the like.

When was the last time you were told NOT to spend money? With new marketing channels popping up on a regular basis, it can be tempting to chase after the shiny new toy right away. But not every marketing tactic is best for your business. Consider Twitter, for example. Everyone is eager to be a part of it, but the truth is that many businesses still aren't seeing any quantifiable value from it -- especially when they consider the lack of a return on the investment that's required just for them to maintain an active Twitter presence in the first place.

A good agency looks for the online marketing mix that's most directly aligned with your company's specific objectives, budget, and needs. If your agency says "yes" to everything, they may have dollars in mind -- not your best interests -- at heart.

How often do you hear from your account manager? If you're a smaller company with fewer marketing dollars, chances are you could get lost in the shuffle. You should hear from your account manager at least twice a week with an update on current activity and results. And of course, you should be able to reach anyone on your account team at any time, with a quick response to any request. If this isn't happening, you might not be getting the same level of performance, resources, time, and effort as other clients.

How transparent is your monthly bill? Commissions and fees often play a large role in the world of online marketing, especially if your strategy requires working with affiliates and other partners. Are you being charged for ad serving? And if so, is your rate marked up? Some agencies charge commission, others don't --and what's important is that you know if yours does. Your bill should clearly outline any and all charges, commissions, and fees up front. If you're not sure why your bill is what it is, ask. You're entitled to an explanation.

At the end of the day, transparency is the key to a successful relationship with your agency. With open communication, clear objectives, and an honest look at activities and results, you're sure to see tangible benefits from your online marketing initiatives.

Keith Kochberg is the CEO of iMarketing LTD.