Some clients will have more knowledge than others and will be able to do some SEO themselves. The reason they seek out SEO people is because with their best efforts they have failed to boost rankings and so employ someone to analyse the site with the aim to boost rankings in serps and sales as a result. Also reducing advertising costs due to better serps returns then the SEO will have done their job.
The main thing I would ask from a client is:
Who are your main competitors?
Why - if you dont know their competitors then you cannot perform SEO to out perform their competitors unless you get lucky. Their competitors may have employed an SEO who has brought their serps to top rankings - narrowed keywords, restructured keywords/phrases, edited page content and such like.
The list of the main competitors will give you some basis for comparison without ripping the exact same information from their competitor sites - as there could also be many offpage factors that determine the competitors better serps results.
For keyword preference - some clients will be adamant that they want some words included - the list will always be incomplete - SEO will refine the list and improve it.
It would be good to have an SEO history plus advertising history... what advertising means did they employ... did they or are they getting penalised from any current advertising methods / tricks to artificially boost rankings.
If you know every advertising provider they approached you can do the research on those companies and check for any adverse ratings.
I am not an SEO, I wish I was as I find the basic SEO a very interesting topic - I just lack the finances & programming skills to develop my own analytics tools or hire someone to create the tools

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