The regular metric for monitoring a website's indexability progress on the web is:
This tells you how many pages of your website Google has indexed, which is a pretty useful figure, because if your site has 500 pages and Google has only indexed 10 - well, you know you need to do something.
But that is what the figure is; it's more of a check on your site's on-line status than how well your site is performing.
So how can we tell how well a site is performing - naturally before sales figures and the like, this is performance via pure web metrics?
Well, by knowing how many different keywords are used to access your site through 'search', is a clear indicator of how accessible your site is to the search engines, meaning, how good the semantics within your content is to a search engine spider. If your content is rich, that is, it makes sense and isn't only jam packed full of keywords, it will be easily indexed and everything written will have a chance of being accessible to a potential customer when carrying out a search in their chosen search engine.
So how's it measured?
Google Analytics makes it so.
When in Analytics go to: 'Traffic Sources', then drill down through 'Search Engines', then 'Google' (Bing and Yahoo! are still miles behind and hardly worth monitoring), the metric for 'keywords' is the default, see Below:
Then it's crystal clear; columns with the keywords listed in order of usage (you naturally get an overall figure at the top & bottom of the page), with the usage for each individual keyword in the first column, then the usual Analytic metrics.
By monitoring this data on a monthly basis you are going to be offering your clients a much more worthwhile piece of data than the site index could ever achieve, and if it increases incrementally month on month, then you're going in the right direction and your clients should be pleased.
Then all you have to do is encourage these new found visitors to purchase, sign-up or whatever your website's goal is...