Check if items exist in the current language?

To see if there is a version of the current item you can do this: Sitecore.Context.Item.Versions.Count > 0

[updated for comment]

I don't claim that this is the most efficient way to determine if an item has a version in a language, but this will work:

bool hasVersion = HasLanguageVersion(Sitecore.Context.Item, "en");

private bool HasLanguageVersion(Sitecore.Data.Items.Item item, string languageName)
{
    var language = item.Languages.FirstOrDefault(l => l.Name == languageName);
    if (language != null)
    {
        var languageSpecificItem = global::Sitecore.Context.Database.GetItem(item.ID, language);
        if (languageSpecificItem != null && languageSpecificItem.Versions.Count > 0)
        {
            return true;
        }
    }
    return false;
}

You can retrieve a collection (LanguageCollection) of an items content languages (ie. the languages for which the item has content).

  LanguageCollection collection = ItemManager.GetContentLanguages(Sitecore.Context.Item);
  foreach (var lang in collection)
  {
      var itm = Sitecore.Context.Database.GetItem(Sitecore.Context.Item.ID,lang);                  
      if(itm.Versions.Count > 0)
      {
          Response.Write("Found language " + lang + "<br />");
      }
  }

Hope this helps :)

NB: Add a comment dude.. please dont just make random edits to my answer. This is the height of rudeness.

Edit: Correcting .. Turns out the method doesn't take into account versions of that language existing.---

to clarify, ItemManager.GetContentLanguages does not get you the list of languages on a given item. It gives the list of all languages you have opted to include in your environment. Under the hood, it does 2 things (based on decompiled code for sitecore 7.2):

1. it calls LanguageManager.GetLanguages(item.Database));
2. it adds to this any languages not already added by step 1 by calling item.Database.DataManager.DataSource.GetLanguages(item.ID);