How can we know that a particular command invokes a new shell or subshell?

I am going through the learning material and can see mention about new-shell or sub-shell being created by a command. My questions is how do we know when that happens, do we have nay defined list of commands that start a new shell.
I am asking this, since I may be looking at set of commands or a script and want to know at what places the new-shell was created and when that new-shell was exited returning the control to parent shell. Please help.