Making sure that techniques implemented by your SEO provider comply with the Google Guidelines for good acceptable SEO is imperative. Have you read these guidelines? Do you know what is considered good practice, versus what is likely to get your site banned by the big boys? Take a look at the guidelines here:
Just to clarify something though before you spend all day and all night checking your site against these terms of service, remember that I have never come across anybody who has any clear understanding of what is good, bad or indifferent. Although the Google Quality Guidelines clearly state what is acceptable and what is not, there are millions of sites out there that bend or blatantly break these guidelines, and still enjoy great rage rankings.