This question is the motivating issue for a considerable amount of research which is taking place around the world today.
Smalltalk testing is complicated.
One must be concerned with ensuring that the testing cover both domain models and graphical user interfaces. There are multiple perspectives to the topic of testing - ad hoc, acceptance, regressive, white box, black box, component, system, etc. I propose that part of the answer to this question must include dynamic software metrics based on coverage. If used properly, in conjunction with other software quality tools and good coding style, coverage can provide a reasonable quantitative indication of the quality of a defined set of tests. The value of coverage testing has been hotly debated for a number of years.