Unity3D: a class which checks if a tag is defined. also could create new ones or remove them.

TagManager.cs

public static class TagManager
{
    private const string TAG_MANAGER_PATH = "ProjectSettings/TagManager.asset";

    public static bool ContainsTag(string tag)
    {
          SerializedProperty prop;
        if (GetAssetDatabase(out prop))
        {
            for (int i = 0; i < prop.arraySize; i++)
            {
                var element = prop.GetArrayElementAtIndex(i);
                if (element.stringValue.Equals(tag))
                {
                    return true;
                }
            }
        }
        return false;
    }

    private static bool GetAssetDatabase(out SerializedProperty prop)
    {
        var db = AssetDatabase.LoadAllAssetsAtPath(TAG_MANAGER_PATH);
          if (db != null && db.Length > 0)
        {
            var so = new SerializedObject(db[0]);
            prop = so.FindProperty("tags");
            return prop != null && prop.arraySize > 0;
        }
        prop = null;
        return false;
    }
}