Why? The Why of it affects the answer as to how to use the SCREEN or other functions.
If you just want to detect if it is your MacBook Air or your MacBook Pro, use the %MacUUID% text token.
You can get the local MacUUID onto the clipboard with this action:
Or, if you really want to use the SCREEN function, you can use the calculation SCREEN(Main,Width) > 1600.
But the real question is why do you want to do this? Because maybe you don't really need to know which Mac your on, maybe what you need to know is the size of the screen. For example, you could move the mouse to the center of the screen like this:
But that is an inefficient way of doing it when you can just do this:
which would work on any Mac and any screen size.
Similarly, if you want to position a window on the Main screen, you can use the SCREEN function to position it regardless of the screen size.
(that example is just the "Move and Resize ➤ Full Screen" option, though its clear you can adjust the details as desired).