If you've seen someone promoting the offer you can always trace the http headers and see where it redirects through. It might go through something like adsjmp.com, cpaclicks.com, etc before it gets to the offer. Then do some research on that jump link and see if you can see what network it's related to.
A good tool for tracing headers is something like "Live Http Headers" which is an add-on for firefox. At least that's my favorite. I couldn't live without it.
Do you have to have a plugin installed for this to work.Yep, also in Firefox 3, you can click "Show All History" and see the jump links as well.