Protocol features and extensions are commonly published in operational attributes of the Root DSE, namely supportedFeatures, supportedControl, and supportedExtension.
These operational attributes can be read using ldapsearch(1):
ldapsearch -b "" -s base supportedFeatures supportedControl supportedExtension
Note that, per the LDAP technical specification, operational attributes are only
returned if requested. Some servers may support the "+" wildcard (which requests the return of all operational attributes).