All rights are a matter of contract and nothing more(or less). One can acquire the rights to produce a revival on Broadway within a stated time period by entering into a contract to that effect with the rights holder.
That contract may or may not be public information. So if you are not finding anything about those rights by googling, there is no resource available to find proprietary information.
Theatrical Index lists only shows that have been registered.
Negotiations relating to Broadway rights are almost never handled by licensing organizations; they are handled directly by the rights holders (usually through their agents and/or lawyers). But when Scott Rudin acquires the rights to produce a revival, he most likely discussed it either with the author or the author's people, not Samuel French or the like. Sometimes the contact may be made through a third party, such as when a show is discussed with an actor, who is friends with the author, and the actor calls his friend to pitch the idea first.
In answer to the original question, if the information cannot be located through googling, you should contact the author's agent, who probably will not tell you who holds the rights but will certainly tell you if anyone has them or if they are potentially available. I say potentially because oftentimes the rights may not be available because they do not want a production at a certain time for whatever reason, and also because there are very few people who will get those rights, even if they have the money for the option, because the author is essentially investing in the ability of the optionee to mount a successful production.
You can usually find an author's agent by googling. A good one stop shop for things is doollee.com