There are two kinds of international assessments. One type is assessing countries with quantitative methods (encounters with corruption, ratings, etc.). Examples of that are the various indexes and polls of Transparency International , the Worldwide Governance Indicators , etc.
Others are more in-depth, looking more accurately at the existence and implementation of some legal provision or policy as well as its compliance with international conventions. Examples of this are GRECO , OECD , UN , European Commission , all of them assessing Estonia from time to time from the viewpoint of some aspect of corruption and preparing comprehensive reports. A country is usually assessed by representatives of some other country and vice versa, so Estonia also assesses other countries in delegations.