Damn easy.

Step 1: Run a linkdomain command at Yahoo! with the following syntax; "linkdomain:url.com -site:url.com", and record the # of results (sample for SEOmoz - 7810)

Step 2: Run a linkdomain command at Yahoo! with this syntax; "linkdomain:url.com -comment -comments -forum -reply -site:url.com" and record the # of results (sample for SEOmoz - 1770)

Step 3: Subtract Step 2 from Step 1 and divide by Step 1 - this is a rough percentage of non-forum / non-blog links to the site. (For SEOmoz 7810 - 1770 = 6040 / 7810 = 77.33% - no wonder we were in the sandbox forever...)

In this manner, it can be shockingly simple to see just how much a site relies on blog comments and forum links for link power. And, if I can run these reports, so can the search engines...

There's one important thing to keep in mind, however. If you are checking the stats for a blog itself, you'll find that a lot of the links are naturally coming from other blogs. A closer look at the results from a reverse of Step 2 (using each term individually in a linkdomain commnad) would then need to be used to determine how spammy or unspammy those links were. Also note that these numbers can be misleading on sites with very few links - it's better to have higher numbers and older sites to check on.