Oh, I see now, please drop my recent post. I realized it from your other question: You are not really using QuickOPC.NET assemblies, you are using Interop to QuickOPC-COM. That's the approach that works, but it somewhat clumsy in terms of typecasting needed, as I will explain in the answer to your other questions.
I got confused because your questions are in the QuickOPC.NET forums (yes I understand, because SQLCLR is a .NET host), but from our perspective, it is QuickOPC-COM product and component that you are using.

May I ask you whether/how you have managed to get EasyDAClient working in SQLCLR? Because, if I remember well, when we attempted that, we ran into problems with the fact there are internal assemblies that are mixed-mode (contain native code), and SQL server refused to load them.

Hello.
We do not have dedicated methods to check the validity. But there are specific error codes returned in such cases. You already know the error code for an invalid OPC server class . Then, there are two such errors for invalid OPC items: One is when the syntax is totally invalid, and one is when the syntax is valid but still the item does not exist. The codes for this are:
//
// MessageId: OPC_E_UNKNOWNITEMID
//
// MessageText:
//
// The item ID is not defined in the server address space or no longer exists in the server address space.
//
#define OPC_E_UNKNOWNITEMID ((HRESULT)0xC0040007L)

//
// MessageId: OPC_E_INVALIDITEMID
//
// MessageText:
//
// The item ID does not conform to the server's syntax.
//
#define OPC_E_INVALIDITEMID ((HRESULT)0xC0040008L)

Why do you want to specifically test these? (because there are other kind of errors that can also cause failures). Also note that the component does re-tries automatically (this is mainly relevant with subscriptions; if you pass in an invalid info, it may become valid, such as in case of server reconfiguration; but you do not have to put in the retrial logic, it's all handled internally by QuickOPC).

You have a typo. It is not "OPCLabs.Kiterver.2", it should be "OPCLabs.KitServer.2".
Kind regards,
Zbynek Zahradnik