Keep tabs. See what people are pinning from your blog by going to: http://pinterest.com/source/YourBlogURL
Do a reverse image search of your jpeg art images.
You can do a reverse image search using TinEye http://www.tineye.com
This little kid explains how to do a reverse image search–
If you have found your art used by others without your permission, here are some organizations that can help–
Join Creative America united against content theft organization http://www.creativeamerica.org
Defend Art http://www.defendart.org
Legal Art http://www.legalart.biz