In principle you should right-click on the block, select "Look under mask". This opens a window with the scheme of the masked block. In the scheme there may be the S-function block, you are looking for.

However, it may be possible the wohle thing is realized with a scheme and only a part done as a S-function, or even without any S-function.

If the block belongs to the Simulink library, normally the code is compiled and it is not possible to disclose it.

This block  belongs to Simscape,attached here are the pictures of the generator SIMULINK model.The figure with name ASM is the actual Asynchronous Machine simulink model and when i look  under mask ,i found this "mask model "  shown in figure 2,and i right click on it ,i can not look under mask ,so i think it is build by building S-Function.when i double click on it ,i can see only this dialog box,shown in figure 3 with name PMC

I thought, you were speaking about Simscape. The library of Simscape consists of objects and not of blocks. Simulink in a block-based simulation tool, i.e. blocks are defined as inputs-states-outputs models. Hence, you organize your simulation model connecting inputs of a block with outputs of the other block. Simscape is a component-based tool. The connections between components are carried out by defined "connectors" or "ports", which are not necessarily inputs or outputs (e.g. you connect a resistance and a capacitor by the pins, but not by voltage and current). When the simscape model is ready, you press play, then the compiler takes the component-model translates it in a inputs-states-outputs representation and generate the S-function that at the end is executed. Thus, you never will be able the see the code behind the object because it does not exist during the edition time. It is automatically generated at run-time.

@Adrian Gambier thank you so much for your valuable answer..