Measure of review bias
There aren’t many ways for your to evaluate how your reviews stand against others, but there is at least one. You can keep a tab of the papers you have reviewed, and look for them a year or so after the review. See if your reviews correlate with the publication (or lack thereof) of the papers in the respective journals… if papers you have suggested to reject are frequently accepted by the editor, then you give typically more negative reviews than other reviewers of your community.

Of course, this is a very black-and-white picture (rejection/publication). But, I don't think you can actually do much better, because “negativity” can hardly be measured quantitatively.



PS: I wouldn't call it a reviewing bias. A review is the editor asking you your frankly assessment of a publication. Toning it up or down because you fell that you do not conform to the “mean” of your field would probably diminish your value as a reviewer. If the editor does not think your reviews are helpful to him, he’ll just stop sending you paper to review! 

I'll finish with an anecdote: there's a journal for which I have recently given a series of quite negative reviews (well documented, and justified in my eye), and I have actually received some positive feedback about my reviews from the editor.