A thoughtful moment in the Arrange Displays pop-up in macOS’s Settings when you choose to designate a screen to be a primary screen:

It not only shows a nice preview of where your menu is going to land, but also changes the shape of the menu proxy you’re holding, to help you connect the two.

(The whole interaction is probably obsolete, though – if I remember correctly, it used to be that you could only have a menu on one designated screen. Today, the menu is replicated on all screens.)