On the Simulink toolbar, select "Display" - "Sample time" and check "all".
This will show the sample time for all of the signals in the model and show you where there is a continuous signal. You can change this by setting the block properties from which the signal orriginates.
Another option is to set the properties of the entire model solver to be "discrete".
This is done under "Simulation" - "Model Configuration Parameters" - "Solver"