Ways of working this out using Google:

Number of optimised pages i.e. sites that target certain subject areas or keyphrases like 'online pr'

allintitle

You can use a google search perameter called allintitle: which shows the number of pages where the title tag has been optimised to carry the name of the keyword you are interested in.

Example: http://www.google.com/search?hl=en&lr=&as_qdr=all&q=seo+intitle (intitle:"online pr") 22,400 results

intitle

or you can go a bit further and add the keywords in the body of the article - this is good since it accounts for fully optimised pages. the search would look like this:

Example: http://www.google.com/search?hl=en&lr=&as_qdr=all&q=online+pr (online pr intitle:"online pr")  22,500

inanchor

Number of sites that come up with a phrase like 'online pr' and which have link text saying 'online pr'. This is interesting because Google puts a lot of dependence on links and what the link text says

Example: http://www.google.com/search?hl=en&lr=&as_qdr=all&q=online+pr (online pr inanchor:"online pr") 14,500 results.

Google trends

Google trends will show you how often a keyphrase is searched for, whilst it does not give absolute numbers, but it still gives you a comparison against other phrases. it also breaks the informaiton down by reigon and it will show you the relative number of times a phrase has been referenced in the news.

The rule of tumb is : bigger the search volume and the greater the number of references, the more difficult it is going to be to get visability.

(As you see here, the term 'online PR is gaining popularity)