js/leaflet.markercluster.js
function (layer) {
    if (!layer) {
      return false;
    }

    var i, anArray = this._needsClustering;

    for (i = anArray.length - 1; i >= 0; i--) {
      if (anArray[i] === layer) {
        return true;
      }
    }

    anArray = this._needsRemoving;
    for (i = anArray.length - 1; i >= 0; i--) {
      if (anArray[i] === layer) {
        return false;
      }
    }

    return !!(layer.__parent && layer.__parent._group === this) || this._nonPointGroup.hasLayer(layer);
  }
function (layer) {
    if (!layer) {
      return false;
    }

    var i, anArray = this._needsClustering;

    for (i = anArray.length - 1; i >= 0; i--) {
      if (anArray[i] === layer) {
        return true;
      }
    }

    anArray = this._needsRemoving;
    for (i = anArray.length - 1; i >= 0; i--) {
      if (anArray[i] === layer) {
        return false;
      }
    }

    return !!(layer.__parent && layer.__parent._group === this) || this._nonPointGroup.hasLayer(layer);
  }
Returns true if the given layer is in this MarkerClusterGroup