Of course it was not until other clients started using our server that we could verify that our interpretation of the parameters and our BER encoding were correct. For this the Z39.50 implementation testbed that Clifford Lynch organized in 1992-1993 was invaluable. Some third-party testing was also done at that time, but it was limited by the lack of availability of good test environments. I continue to seek out clients as they are developed and to run them against our server. For the last three years, RLG has made its server and its very large databases available to client developers, because of our commitment to standards development in general and to encourage the widespread use of Z39.50 in particular.
People find it very robust.
I am not aware of any formal testing tools. Developing common tools or a generic test suite has come up in the past, but it's such a large job that the ZIG always shies away from taking it on.