This will switch to an indexed layout (3 in this case) and print to chat which one it’s using:
/run EditModeManagerFrame:SelectLayout(3)
Replace 3 with different numbers to see options (1 is Modern, 2 is Classic for me, and new ones start at 3).
Also if the above has issues with taint for whatever reason, you can use the base API too:
/run C_EditMode.SetActiveLayout(3)
Though that won’t tell you which one you just loaded.
edit: Here’s a macro to toggle between layouts 2 and 3:
/run local a,b=2,3 EditModeManagerFrame:SelectLayout(C_EditMode.GetLayouts().activeLayout==a and b or a)
I’m not seeing anything obvious that lets us deal with layouts by name, but I haven’t looked hard so they may be there.