This paper studies the situation in which the system under test and the system model are distributed and have the same structure; they have corresponding remote components that communicate asynchronously. In testing, a component with interface $$C_i$$ has its own local tester that interacts with $$C_i$$ and this local tester observes a local trace consisting of inputs, outputs and durations as perceived by $$C_i$$. An observation made in testing is thus a multi-trace: a tuple of (timed) local traces, one for each $$C_i$$. The conformance relation for such distributed systems combines a classical unitary conformance relation for localised components and the requirement that the communication policy was satisfied. By expressing the communication policy as a constraint satisfaction problem, we were able to implement the computation of test verdicts by orchestrating localised off-line testing algorithms and the verification of constraints defined by message passing between components. Lastly, we illustrate our approach on a telecommunications system.