When you create an AD realm on FDM and add it to the VPN config, it essentially creates a aaa-server config like on the ASA as seen below:
aaa-server AD host 192.168.1.1
server-port 389
ldap-base-dn dc=domain,dc=com
ldap-scope subtree
ldap-login-password *****
ldap-login-dn administrator@domain.com
server-type auto-detect
You should be able to run "debug ldap 255" on the CLI to see debug logs when testing. This should look like the regular LDAP debugs on the ASA as seen here:
https://community.cisco.com/t5/security-documents/remote-access-vpn-on-asa-authentication-using-ldap-server/ta-p/3144864