string layerName = "My layer name";
if (MapApplication.Current != null && MapApplication.Current.Map != null && MapApplication.Current.Map.Layers != null)
{
     LayerCollection layers = MapApplication.Current.Map.Layers;
     while (layers.Any(l => MapApplication.GetLayerName(l) == layerName))
     {
           Do what you want here ....................
     }
}