How do I know if my SEO guy is doing the right thing?

[Doodled]

If he carried out Black Hat links your site will probably be penalised within the next few weeks and you will loose your rankings. That's a hard way to find out.

But like with any trade do not hire a person to do a job unless you have carried out some common sense checks first:
- ask who his past customers are and contact them for references
- ask what keywords he himself ranks for (and check!)
- search him on the internet for any mentions - good or bad
- learn a but about SEO (just a bit) so you can ask him questions and consider his answers.
- be aware that links are a very small part of SEO

Have a look at the following page to understand the differences between Black Hat and White Hat SEO better.