How to check if an appSettings key exists?

if (ConfigurationManager.AppSettings.Settings.AllKeys.Contains("myKey"))
{
    // Key exists
}
else
{
    // Key doesn't exist
}

MSDN: Configuration Manager.AppSettings

if (ConfigurationManager.AppSettings[name] != null)
{
// Now do your magic..
}

or

string s = ConfigurationManager.AppSettings["myKey"];
if (!String.IsNullOrEmpty(s))
{
    // Key exists
}
else
{
    // Key doesn't exist
}