How to test a LDAP connection from a client

Use ldapsearch. It will return an error if you cannot query the LDAP Server.

The syntax for using ldapsearch:

ldapsearch -x -LLL -h [host] -D [user] -w [password] -b [base DN] -s sub "([filter])" [attribute list]

A simple example

$ ldapsearch -x -LLL -h host.example.com -D user -w password -b"dc=ad,dc=example,dc=com" -s sub "(objectClass=user)" givenName

Please see this link: http://randomerror.wordpress.com/2009/10/16/quick-tip-how-to-search-in-windows-active-directory-from-linux-with-ldapsearch/

Edit: It seems you don't have pam configured corectlly for gdm/xdm here is an example how to do it: http://pastebin.com/TDK4KWRV

To know if my server and clients settings are correct I use this:

ldapsearch -x -b "uid=username,ou=people,dc=example,dc=com"

the answer will be something like this on success:

# extended LDIF
#
# LDAPv3
# base <uid=username,ou=people,dc=example,dc=com> with scope subtree
# filter: (objectclass=*)
# requesting: ALL
#

# username, people, example.com
dn: uid=username,ou=people,dc=example,dc=com
cn: User Name
uid: username
uidNumber: 1050
loginShell: /bin/bash
homeDirectory: /home/webminder
gidNumber: 1030
objectClass: posixAccount
objectClass: shadowAccount
objectClass: person
objectClass: inetOrgPerson
gecos: User Name
sn: User Name

# search result
search: 2
result: 0 Success

# numResponses: 2
# numEntries: 1

you can use different filters. I only have one server on my network

Your problem is not LDAP, It's PAM.

As noted in the comments on Sacx's answer you probably do not have the console login application (usually the PAM system, xdm, gdm, etc. service(s)) configured to consult LDAP for authenticating users.

You should review the PAM documentation for more information on how to set this up.