function sample() {
  var result = sforce.connection.describeGlobal();
  var sobjects = result.getArray("sobjects");
  for (var i=0; i<sobjects.length; i++) {
    if (sobjects[i].name == "CustomObject__c") {
      alert(sobjects[i]);
    }
  }
}

//取得できるプロパティと値

//{activateable:'false', createable:'true', custom:'true', customSetting:'false', deletable:'true', deprecatedAndHidden:'false', feedEnabled:'false', keyPrefix:'a0B', label:'カスタムオブジェクト', labelPlural:'カスタムオブジェクト', layoutable:'true', mergeable:'false', name:'CustomObject__c', queryable:'true', replicateable:'true', retrieveable:'true', searchable:'true', triggerable:'true', undeletable:'true', updateable:'true', }