Normally, the Organisation for the Prohibition of Chemical Weapons (OPCW) carries out inspections when there is a suspicion of chemical weapons being used.
If their assessments coincide then it is considered sufficient proof to go to the OPCW executive council where a state being criticised could challenge the evidence.
Two or three of them would be selected to analyse the samples, together with a set of controls, which would be prepared by the OPCW Central Laboratory, to ensure the quality and dependability of the results.
For this off-site analysis, the Organisation for the Prohibition of Chemical Weapons (OPCW) - the implementing agency of the Chemical Weapons Convention - has established an international network of highly competent so-called "designated laboratories" that routinely undergo proficiency testing to ensure that their standard of analysis is up to the needs.