In model-driven engineering (MDE), models constitute the main development artifacts. As a consequence, their quality significantly affects the quality of the final product. Thus, adequate techniques are required for ensuring the quality of models. We present a testing framework, comprising a test specification language and an interpreter, for validating the functional correctness of UML activities. For this purpose, we utilize the executability of a subset of UML provided by the fUML standard. As UML activities are employed for different purposes, from high-level process specifications to low-level object manipulations, the proposed testing framework not only allows to validate the correctness in terms of input/output relations, but also supports testing intermediate results, as well as the execution order of activity nodes. First experiments indicate that the proposed testing framework is useful for ensuring the correct behavior of fUML activities.