This is not an easy task. The market is unregulated and saturated with unqualified offers. The first thing you'll need in your personal arsenal is...
If you don't understand the jargon, ask your potential SEO to explain it. You'll find most dodgy operators simply don't have the patience to answer your many questions enough for you to understand what they might be planning to do.
Local knowledge is also critical if your potential customers are going to be typing in a search that uses location names. We've seen too many sites supposedly 'optimised' for Sydney SEO but with content that was obviously written by someone in Asia. Use local knowledge for local search results, and preferably someone who has a track record achieving results in your same location.
Check out any testimonials for the company you're considering. Contact them yourself, careful to note any relationship with the SEO company other than being a long-standing client. A quick email or simple phone call can save and make you $$$s.
Again, check the location: are they in Sydney? Were their visitors searching for generic search terms (without a location) or where the results based on searches that named the location? Competition is much greater in capital cities, and especially so in Sydney.
Don't trust any success examples unless you Google them yourself. And don't trust the links to a Google search provided by some companies as they can be twisted! Go to www.google.com.au yourself, and type in the search term. Then see if you can find the client they're bragging about, and make sure the website actually links back to them as proof.
Try it with 'wholesale skylights' and see our client Twista Skylights. And 'persian rug washing' for Oriental Rug Care. Or in Sydney there's 'Sydney gutter cleaning' and our client GutterClean.com.au. (This is an incredibly competitive market and they've been tops for years now.) Same with the saturated market for 'window cleaning sydney' and our double hyphen domain client sydney-window-cleaning.com.au! Same with 'Wollongong solar panels' for SolarPowerPro.com.au, and plenty of others...