Navigation
Have you run a link-checker over the internal links?
Inbound Links
How many IBLs does the site have and from where?
Outbound Links
How many OBLs does the site have and to where? Are they what could be described as "bad neighbourhoods"?
Domain Check
How new is the site? Was the domain recently registered and if so, for how long? Was the site built just recently from scratch, or was there a holding page there until recently? Most important, did the domain used to belong to Bad Guys and could be blacklisted?
Server Check
Any errors here? Is the site visible to the engines?
Spider Test
Is the internal navigation of the site traversable by the engines' spiders?
Doc type Tags
Is there a Doc type Declaration present?
Title Tags
Are the keywords present in the title tags and mentioned first?
Title Tags
Are the title tags actually present? You'd be surprised how many sites don't have them!
Meta Tags
Is the description tag present and properly filled (not stuffed!) with keywords?
Charset
Is the character set declaration present? Is it present before the title tag where it should be for optimum page loading speed?
External Javascript Tag
Are Javascripts (if any) present in the page or are they referenced in an external file?
External Style Sheet
Are CSS styles present in pages or are they referenced in external style sheets?
Code to text ratio
Is your text content greatly outweighted by the page's code? Check it with Code to Text Ratio Tool
Company Address
Is the (physical) address of the company present on the pages?
Privacy Policy
Do you have one that the world can see?
Spell Check
Before you can run any kind of density check, you have to ensure that key words and phrases are correctly spelled.
Keyword Density
What's the keyword density of the page? Will the engines be able to ascertain what the page is about from the keyword density alone?
Sponsored Links
Is the client company already using key phrases in sponsored links?
H tags
Are keywords present in the header tags?
Text Content
Is the relevant key phrase appropriately sprinkled throughout the page's text content?
Adsense Relevance
What does the Adsense Preview Tool at Adsense Preview say about the page? Do you get ads related to what you like to think your content is about?
Compressed Graphics
Do the site's graphics already have compression applied?
Internal Linking Strategy
Is the site already using appropriate keywords in its internal links?
Robots.txt
Is there a Robots.txt present?
Does the Robots.txt actually work?
Is it properly configured?
Custom 404
A customised 404 Error page can be a useful addition to any site's internal linking and branding strategy - does the site have one?
Site Map
Even the smaller sites can benefit from a site map - does the site have one?
Canonical Issues
Does the http://example.com/ url resolve to the http://www.example.com/ url? You could be losing rankings if it doesn't!
Cache Check
Is your site properly cached by Google? You'd be surprised by what this check can show up.
Text Cache Check
How does your site look in Google's all-text version of the cache? Again, you'd be surprised by what this can show up.
Graphic File names
If you have a whole load of graphics on your site, are you making the best use of their file names? Would someone be able to find your site through Google Images?
Graphic Placement
Image search is taking on more and more importance - are your images placed close to relevant text? Are they captioned?
Google Sitemaps
Is there an XML Site Map in place? Is it accurate? Google, Yahoo and MSN all expect them now, so it needs to be there.
Google Analytics
Does the site already have decent stats, and if not, will they consider implementing the increasingly useful Google Analytics?
Validation
Every site should validate, or get as near to it as is feasible.
Spamming
Are they spamming? Hidden text, invisible links, anything at all that could potentially cause the site problems?
Spamming using CSS
Are they spamming using CSS? Bit more complex this one... is there anything untoward in the external stylesheet?
Accessibility
Does the site satisfy basic accessibility criteria? This is slowly becoming more important.
CMS, or Content Management Systems
Does the site use a CMS? In my opinion, many of these are useless for their stated purpose and shouldn't be on the market. Many simply can't be optimised as you'll never have access to the code in order to be able to alter it. This is something that normally can't be established without lengthy and time consuming (read: expensive) correspondence with the CMS developers. In some cases, the only real solution is to scrap the site entirely and begin again with fresh and accessible code - not the most popular of options with anybody (except lawyers)...
Own I.P.
Are you sharing a server with bad neighbourhoods, or do you have your own (and therefore exclusive and beyond contagion by association with the spammers) I.P. address?
Funnelling Page Rank
Is your site in the top ten in Google for 'Terms & Conditions'? Oh dear - better think about using the rel=nofollow attribute to steer PR away from your non-essential pages towards your important ones.
Duplicated Content
Maybe the site's deliberately ignored because, essentially, the content's already in the index - on an existing site. Check for this at Copyscape.
Page Loading Time
Is your site responsive or slo-o-o-o-w? If it's slow, and your competitors' are quicker, guess which Google will be ranking higher? Better check it over with Yslow.
How easy is it to read?Readability testing will tell you.
If you have to be a rocket scientist to understand it you'll probably be going over the heads of your target market.
Dynamic sites with proprietary code
Some sites are built in .asp or .php (or whatever), that simply can't be optimised because that functionality hasn't been built into the code. See above.
Still stuck? Still got that uneasy feeling that something's simply not jake?
Download the Paessler Site Inspector and integrate it into your browser. Run all the tests it has in it. That's right, all of them.
Still stuck? Goodness...
How about you head on over to the Faganfinder "Urlinfo" section, click the "Develop" tag and run all the tests it has there. That's right, I did say all of them! Or at least until you find something that shouldn't be there.
Nikita The Spider
I run a difficult site through this online tester; online web site tester. I can't tell you what to look for though, not in any detail.
DNS Problems
Could be something lurking in there somewhere that makes the engines wary... try
Optispider, downloadable from Optispider
Essential but not for the uninitiated because as is the case with Nikita's Spider mentioned above, without the background that I've got (or similar) you probably won't understand the significance of what you're seeing in the results.
Absolutely no joy?
Call me!
Most important of all - Keyword Research.
This is separate from all the above. It's probably the most important of all the items on any SEO Checklist. I'll be publishing further details of my Keyword Research Service shortly.