determine if pdf has layers

I'm trying to determine if a pdf has layers (I'm very new to Javascript and this API). Below is the function that I've written for this:

var check_flat = function()
{
    for(var page = 0; page < this.numPages; page++){
        var ocgArray = this.getOCGs(page);
        if(ocgArray != null){
            console.println(page + " has layers");
        } else{
            console.println("no layers on this page");
        }
    }
};

When I try this on a pdf that I'm certain has layers, I only get 'no layers on this page'. Your help is very much appreciated!