To determine whether a queue is storing any entities, use this technique:
Enable the n output signal from the queue block. In the block dialog box, on the Statistics tab, select the Number of entities in block, n check box.
From the Sinks library in the Simulink® library set, insert a Scope block into the model. Connect the n output port of the queue block to the input port of the Scope block.
The scope shows if the queue is empty.