You can use this function to set the y position of the layer within the currently scoped room. You supply the layer ID (which you get when you create the layer using layer_create()) or the layer name (as a string - this will have a performance impact) and the function will move the layer the given number of pixels along the vertical axis of the room.
Argument Description layer_id The unique ID value of the layer to set the y position of y The y position in the room to set the layer to
var lay_id = layer_get_id("Sprites");
if layer_get_x(lay_id) != 0 || layer_get_y(lay_id) != 0
The above code checks the given layer position and if it is not set to (0, 0) then it is set to that position.