How to find if the LDAP server is not rechable

Hi, I've a setup with freeradius-server-2.2.3 and Windows AD 2008 [LDAP server]for authentication. I need to implement fall back feature where if Windows AD server is not reachable , I need to authenticate using local database. The problem I'm facing is : When LDAP server is reachable and giving wrong password result is RLM_MODULE_REJECT. LDAP server not reachable the result is RLM_MODULE_FAIL. But for both the case reply code will be PW_AUTHENTICATION_REJECT. So please guide me in how to differentiate LDAP server not reachability from error due to wrong credentials. Thanks, Adarsha