Unidirectional channel systems can be tested

\"Unidirectional channel systems\" (Chambart & Schnoebelen, CONCUR 2008) are systems where one-way communication from a sender to a receiver goes via one reliable and one unreliable (unbounded fifo) channel. Equipping these systems with the possibility of testing regular properties on the contents of channels makes verification undecidable. Decidability is preserved when only emptiness and nonemptiness tests are considered: the proof relies on a series of reductions eventually allowing us to take advantage of recent results on Post's Embedding Problem.