I agree only partially with the opinions here. As IncredibleHelp states, it should be enough asking the right questions and getting the right answers. Unfortunately, this is not the usual scenario. Most clients don't know how their site is generated and can give very optimistic answers. The
SEO company might end up with a very complicated, sometimes impossible, work to do for which the quote simply is not realistic. Back in 2005, we gained a contract to make the
SEO of a big institutional site in 4 different languages and we crawled the site to make an estimation of the content. We estimated that there were 10.000 pages in each language in what seemed like a normal database driven site. We agreed on producing a unique title and metas for each page. Actually, the site was produced with an XML architecture, no database, and there were 100.000 pages in each language, most of them were hidden behind search forms. So it was a big indexability work and we had to figure out how to generate 400.000 unique, relevant titles and metas and insert them into their code. Nothing close to what we had been able to forecast from seeing the code only.
So, although there are some workarounds, the asking to see the code on the server seems to me pretty reasonable or, at least, justified.