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Some good suggestions above, try some back link checking tools, check their Domain Authority, etc. However, in my opinion, the best way for you to ensure your SEO person is building good links is to learn the basic difference between a good and bad link and actually check them yourself (the bigger your site and the more links you build, the less feasible this is, but the concept that you should be able to look at the links being built and understand what is a good or a bad link is still applicable). Obviously if you are building massive numbers of links, this is difficult (although there are tools that can help), but if your SEO employee (I assume it is singular) is building good links, they shouldn't be building massive numbers of them unless they are coming organically (through creating content or a product that is so popular that high quality links are appearing without traditional link building). Also, how are you measuring success? Ranking growth? Number of links? Quality of links? If you ask your SEO person to report on the links being built and ask he/she to include measures like Domain Authority, Page Authority, etc and then just try and audit the links periodically, you'll start to learn enough about SEO to measure their performance yourself (seriously, try Googling "audit my back links," there's some great tools out there, as well as reasonably simple explanations of the major things to look out for. I also agree with those mentioning that outsourcing SEO is a dangerous (if somewhat necessary) strategy. In my opinion, learning about SEO basics is one of the single most valuable things a small business owner can do, since it will both improve your ability to market online, as well as protect you against hiring a bad employee.