To to list the compiled-in modules for your Apache installation, use the following command

$ apache2 -l
Compiled in modules:
  core.c
  mod_log_config.c
  mod_logio.c
  prefork.c
  http_core.c
  mod_so.c

For older systems running apache 1.x, the command is as the following, and should produce the same output:

$ httpd -l
Compiled in modules:
  core.c
  mod_log_config.c
  mod_logio.c
  prefork.c
  http_core.c
  mod_so.c

For loadable modules, they are normally placed in the modules directory as in the following;

$ ls /usr/lib/apache2/modules/
httpd.exp               mod_cgid.so          mod_mem_cache.so
libphp5.so              mod_cgi.so           mod_mime_magic.so
mod_actions.so          mod_charset_lite.so  mod_mime.so
mod_alias.so            mod_dav_fs.so        mod_negotiation.so
mod_asis.so             mod_dav_lock.so      mod_proxy_ajp.so
mod_auth_basic.so       mod_dav.so           mod_proxy_balancer.so
mod_auth_digest.so      mod_dbd.so           mod_proxy_connect.so
mod_authn_alias.so      mod_deflate.so       mod_proxy_ftp.so
mod_authn_anon.so       mod_dir.so           mod_proxy_http.so
mod_authn_dbd.so        mod_disk_cache.so    mod_proxy.so
mod_authn_dbm.so        mod_dumpio.so        mod_rewrite.so
mod_authn_default.so    mod_env.so           mod_setenvif.so
mod_authn_file.so       mod_expires.so       mod_speling.so
mod_authnz_ldap.so      mod_ext_filter.so    mod_ssl.so
mod_authz_dbm.so        mod_file_cache.so    mod_status.so
mod_authz_default.so    mod_filter.so        mod_substitute.so
mod_authz_groupfile.so  mod_headers.so       mod_suexec.so
mod_authz_host.so       mod_ident.so         mod_unique_id.so
mod_authz_owner.so      mod_imagemap.so      mod_userdir.so
mod_authz_user.so       mod_include.so       mod_usertrack.so
mod_autoindex.so        mod_info.so          mod_version.so
mod_cache.so            mod_ldap.so          mod_vhost_alias.so
mod_cern_meta.so        mod_log_forensic.so

These modules are however only enabled if it’s defined by the LoadModule directive in the apache configuration file.