Have them give you references--companies they have put on the first page of Google, Bing, and Yahoo.
Have them tell you how they will do it. Look on this forum for "black hat" SEO techniques. If the company is going to use "black hat" techniques, RUN AWAY FAST AND FAR! Black hat techniques get you on to the first page of Google, Bing, and Yahoo--for a while. Then the search engines find out about the techniques, and you are NO LONGER on the first page. In fact, if the company did a lot of black hat SEO, your site could be banned.
Have them give you a proposed timeline to get on the first page, as well as a budget for the iniital setup and monthly costs.
Learn SEO yourself, if only to understand when a company is trying to blarney you.
its good, if you hire a team instead of out source the work. Because you don't know what technique they would use to achive the ranking and if they use black hat technique than your site penalized by search engine after some time.
I wouldn't hire anyone who promises to get your site on first page of google, because they don't control Google, and google is making so many changes lately it's getting hard to know what is accepted and what isn't - even some of Google's own old webmaster guidelines can now cause a penalty.