How do I check if there is no tile

I'm making a level maker for a game! Good fun!

The game's tilemap design is top down and so it uses 2 tiles to create a 3D effect.

https://gph.is/2LgP6Sw

I want to make it so that in the editor, when a tile is placed, it is given the 3d effect and the tile below it becomes the 3d slab, but for some reason it works sometimes and not at others.

Of course if there is already a tile under it it won't place the 3d slab.

https://imgur.com/a/JzElfX9

I need to check if there is a tile and then either place one if there isn't or just do nothing.

well, I found an easier way to do it using arrays, but thanks for the help anyways!