The problem of on-line testing of asynchronous circuits is analysed, several infrastructures are proposed and a self-checking tree checker is designed. The checker uses on-demand self-test, which reduces power consumption and guarantees bounded self-test period. Objects under test are tested by observing protocols at their primary inputs and outputs. The checker does not slow down the functional system, as it only samples the signals. The protocols are checked by identifying enabled and refused signal transitions in each state of the system. The fault coverage of internal faults of the checker is calculated. Simulation results are included.