After cold-callers offering to make you a small fortune in PPI compensation, chances are that your biggest source of spam is from a self-styled SEO expert promising to sort out your online ranking.

There’s no question that SEO is the most important part of any digital marketing strategy, keeping the big players at the top of their game and giving the smaller ones the unfair advantage they need to get ahead.

But how can you tell a real SEO expert from a snake oil salesman? Here are some searching questions that may help.

Why are you not the top-ranked SEO result on Google?

The company you choose to do your SEO won’t be the first one listed on Google. Why would you not pick the SEO expert ranked number one in your area? Surely that’s what you want them to do for you.

There are lots of reasons not to trust search engine rankings for this particular decision. If it’s a paid entry, the one’s at the top of the page with ‘Ad’ next to the web address, then it’s obvious how they got there.

Don’t assume they necessarily have deep pockets: they might have blown their entire marketing budget for the year on a single week at the top of the chart.

But there’s a subtler point. The best companies will be busy on fee-earning work for clients not constantly pushing themselves uphill. They will have a full order book and will rely as much – or more – on referrals from previous customers as on advertising. If it’s a smallish business, why are the staff not spending all their energy on the clients.

An unfeasibly good ranking tells you they may have time on their hands.

Serial spammers should be avoided for similar reasons. If you have been worn down by a SEO expert bombarding you with unsolicited emails, don’t read this as a sign of their marketing skill or perseverance. Think instead about what this tells you about the work they will do on your behalf, and how this might be received by their next victims, your customers.

What will SEO do for my business?

Marketing has always suffered from a perceived lack of quality. It creates the conditions for sales, but it doesn’t sell things. Return on investment can be hard to measure or may depend on an act of faith. Sales can be linked to a specific campaign or activity but often without hard proof.

None of this should stop you asking what return on investment you can expect from an SEO provider. Good ones will be able to calculate the price of delivering a qualified lead.

Extravagant claims should set alarm bells ringing. The best companies are those that will decline to work for you because they know they won’t get results or that your expectations are in the wrong place.

You will know you’ve made the right choice if your partner comfortably exceeds the targets set for the first month or two. They said you’d get 100 warm leads. You got 150. Surprise is a much more pleasant emotion than disappointment. A company that sets expectations at a level it knows it can beat already has the secret of making happy customers.

How are we going to keep it going?

All marketing suffers to some extent from the law of diminishing returns. The biggest impact of a new PR strategy is in the first few months, when the messages are fresh and relatively few people have heard of your business. This is why PR is usually outsourced and agencies rarely hang on to their clients for more than a year or two.

SEO is similar. Your provider will have the advantages of building from a low base. After a few months you may find yourself asking what they plan to do for an encore. This question is probably best asked at the beginning of the relationship.

Your ideal provider should anticipate the problem and point out that expectations may need resetting after six months, after which you may either need to buy more of their time to maintain the level of growth you’re seeking or scale back your investment to whatever you can afford.

Does content really matter anymore?

Before the web appeared, content (or what we used to call editorial or programming) was all important. Most of the credible stuff in the newspapers or on television appeared between the adverts.

For a little while it appeared that web search engines had changed all that. Mastery of the technology, expertise in new media and wearing jeans to work every day became the critical success factors.

But to borrow an analogy from the film industry, special effects will only get you so far and ultimately there’s nothing you can do to hide a bad script.

The best SEO solution in the world won’t get you off the hook of coming up with fresh ideas and creating new content.

Increasingly SEO businesses reflect this by offering service that combine high tech elements with other forms of creativity – message development, content creation and traditional media relations. The more comprehensive these services become, the more you’ll need to pay.

What’s the secret of your success?

This is a good question for flushing out two kinds of company you don’t need. If the answer is a jargon-ridden claim that they have a secret algorithm that can fool Google, they are either deluded or dishonest. Nobody can out-Google Google – or not for long. The major search engines pay thousands of people to ensure that their systems can’t be hoodwinked by two teenagers in a bedroom.

Search engine rankings are no longer based on weight – crude repetition of key words; they increasingly rely on a number of measures including the quality of the content being searched. Stuffing pages with arbitrary keywords or hidden text will not only fail to get the desired result, they will push you further down the rankings.

Yes, we’d all like an unfair competitive advantage, but only Google and the other big players have one.

Any self-respecting SEO should readily acknowledge this. Score five for an honest admission that there are several factors in a successful search marketing strategy. Score zero or less for claims of a technological secret weapon.

Arjun Sandhu is CEO of onlinemarketing.org.uk.