The OPC Foundation provides a test harness for OPC Client and OPC Server applications. Vendors use this harness to ensure that their OPC applications can pass all the tests. If the OPC application passes all the tests, the Vendor can state that their application is OPC Compliant. They are also able to post the results of their tests on the OPC Foundation’s website.
You should ask all OPC vendors who state their applications are OPC compliant about the last time that they attended an OPC Interoperability session and the results of the test.