When you view your menuContribution
in the editor, you can add a new command and click browse next to the commandId in the Extension Element Details. You'll get a list of all possible comands (at least in my Juno Version).
If you are looking for a specific item, Alt+Shift+F2, then click on the menu entry you want to inspect.
The menu File -> New however consists of Wizards. You can add to that using the org.eclipse.ui.newWizards extension point and pointing it to your Wizard
implementation.