In what context? Are you trying to use it in a script with GTRR and Get Layout Number by Calculation?
There's also a Get (LayoutNumber) function.
You can also go into Layout Mode and start scrolling thru the book icon, noting the number.
Or you can put Get (LayoutNumber) into your data viewers' watch tab and see what number pops up when you switch layouts.
Neither Get ( LayoutName ) nor Get ( LayoutNumber ) are really "layout ID's" as you can change a layout's name at any time and if you add/remove other layouts, this can change a layout number. But both have their uses as a way to identify the current layout such that it can be saved in a global variable or field so that other scripts can return the user to this layout.