it basically depends on when the creators (all of them ) died, plus a varying number - now about 70 - years.
Google the subject. There are all sorts of sites.
Also please not that extensions can be granted for a number of reasons, so be very careful.
Universal had no rights to "Frankenstein" yet the successfully scared away many adapters with threats of a lawsuit (they came after me for trying to pitch a "Mummy" movie six years before they did the Brendan Fraser versions).
Probably the easiest way would be to pick up your book, note the name of the publisher, and give them a phone call. I'm not sure who you'd ask to speak to once you're through to the publisher, but whoever answers the phone will certainly have a better idea. As you're looking at public domain, you will most certainly be dealing with publishers and maybe estates; if it were modern work, you'd want the author and/or their agent.
Depends on what you're trying to adapt. If it's a novel, the publisher, if it's a movie, the studio. I'm developing a musical based on a play (actually, I'm looking for a composer now lol). I got adaptation rights simply by writing the author, and setting up a meeting to discuss what I wanted to do.
Title 17, Chapter 3 of the U.S. Code, (Duration of Copyright) details the current law. It'd not particularly difficult "legalese":
The biggest problem if a work is not in the public domain is frequently receiving permission from the individual holding the rights. Sometimes the up-front money is prohibitive and at other times permission is not given for many reasons. It took the Steppenwolfe Theater Company a long, long time to convince Steinbeck's widow that "The Grapes of Wrath" should be adapted and that group had a lot of money and clout.
It's a shame. There are so many wonderful works crying out for theatrical adaptations but they'll never see the light of day. But here is the law: