I dont know if we need more, in my place the Toogle button is marked when in 2D, not marked when in 3D, a very simple indicator.
Also the length of inputfields area are longer in 3D as i also get the Z Field in there
All in the Inspector Bar
And if i select one object in 2D my selector have a 1 arm/ 1 green dot, in 3D it have 3 arms/ 3 green dots
I can also go in to Selector Property and say it should not have any Shell if in 3D
So a lot of options, and at least 1 of this you should be able to notice.
V18, V19, V20, V21, 2015/ 16/ 17/ 18 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB