Not sure what you mean by "system call". Commands that you enter in a terminal can be three types (minimum):
Built-in commands
Shell commands
Scripts (can call either built-ins or shell commands)
I believe you can tell a built-in by searching for it in the "executable" directories--eg: /bin /sbin /usr/bin etc. If there is no file to match the command, then it is built in to the kernel.
Note that this definition does not jibe with what I find in "Linux in a Nutshell", so I may be way off base.
For any purpose I can imagine, you will not care if it is shell or "built-in"
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