How to determine whether the shortcut command is triggered by Siri or the user clicks?

There isn't a way to do that, your intent's implementation shouldn't depend on how the customer chose to invoke it. To handle visual vs. non-visual invocations, you can provide shorter and longer dialogs via IntentDialog, so that if a customer invokes your intent on screen, the system will use your shorter dialog because you can also show a visual snippet, but if a snippet can't be shown (such as if the device is currently in use with CarPlay), the longer dialog will be used instead.

—Ed Ford,  DTS Engineer