You can use this function to get the IDs of all layers assigned a specific depth. You give the depth to check and the function will return an array with 1 or more entries depending on whether there are any layers at the given depth or not. If there are no layers at the given depth then the array will have a single entry at the  position with a value of -1, but, if there are layers at the depth, then an entry will be made in the array for each layer found - the entry value will be the unique ID value for a layer.
Argument Description depth The depth to check and retrieve the layer IDs from.
var a = layer_get_id_at_depth(0);
if a != -1
for (var i = 0; i < array_length_1d(a); i++;)
The above code retrieves data about the layers with a depth of 0. A check is done to see if any layers exist at that depth, and if there are then the returned array is parsed and each of the found layers is destroyed.