There are a number of online web-support groups that maintain registries of agencies and user feedbacks.
Keep in mind that an agency that may be great in China, for example, may have limited contacts in Belarus. An agency that has a good reputation in International may not be as good for domestic, and so on. The feedback and capability will vary from country to country, and also vary depending upon the political state of adoption law in that nation AT THE TIME of the reporting parties adoption quest. So, say I adopted from Bulgaria in the late 1990s and had a good experience, the experience of a family adopting now, during the changeover in laws and systems since BG joined the EU, may be very uncomplimentary.
http://www.karensadoptionlinks.com/agency.html
GAARP-Adopt, a website for older adoptive parents adopting from all situations (foster-to-adopt, grandparent adoption, single adoption, international adoption) has many individuals who will likely be willing to write off-list and share their experiences and input.
I'd also suggest checking the Attorney General's Consumer Protection offices for any states involved for feedback.