How to check a jabber id exists on server or not?

There is no direct way to get it ... what i did is extended the community edition code what suite my needs.
you will have to create your custom module to get this done

The workflow will be
send a custom IQ from client with custom elements and namespace .. eg
lit:iq:search xyz
capture it at the ejaberd server
once you capture do the mod_custom_module
odbc_queries:searc_user(user)
select username from iusers wehere username=user
based on result send the result IQ
capture it at client and do your operations.

Thanks
Jeet