This paper studies the problem of testing concurrent systems considered as blackboxes and specified using asynchronous Communicating Finite State Machines. We present an approach to derive test cases for concurrent systems in a succinct and formal way. The approach addresses the state space explosion problem by introducing a causality relation model and the concept of logical time to express true concurrency and describe timing constraints on events. The conformance relation between test cases and trace observed from the real system is defined, and a new test architecture as well as a test case application is presented according to the conformance relation defined. To improve verdict capability of test cases, the approach is enhanced by relaxing the unit-time assumption to any natural number. And a computationally efficient algorithm for the enhanced approach is presented and the algorithm is evaluated in terms of computational efficiency and verdict capability. Finally the approach is generalized to describe timing constraints by any real numbers.