In the end, this has little to do with the company, and everything to do with paperwork.
The laws for periodicals differ a tiny bit in some places, but the general idea is that copyright (when acquired correctly) lasted for twenty-eight years, then could be renewed for another twenty-eight. That's about it. So that means there are three possibilities and some...gray areas.
1. Copyright and renewal have both expires: Impossible for comic books, because renewals became automatic (and longer) in 1978 or so, and only things published prior to 1923 fit the bill.
2. Copyright term expired and no renewal was made: This is the case that involves defunct companies . If you fold and didn't sell the copyright to anybody, there was probably nobody around to renew. You can find good research-fodder starting from here:
http://www.kingkong.demon.co.uk/ccer/ccer.htmFind the publication year. Click on the renewal year(s). At the very bottom of the page, you'll find renewals for the year. So, for example, on this page (1970's renewals, which would've mostly been published in 1942):
http://digital.library.upenn.edu/books/cce/cache/pdrn1970f403.gifyou can see that three issues of "Batman" were renewed (#14-16), but the first issue of Nedor's "Black Terror" book isn't there. You still need to check the surrounding years, in case there was a mixup, but if there's nothing, then "Black Terror #1" is public domain.
3. No original copyright: There is, of course, a second reason that there's no renewal. The book may not have been protected by copyright in the first place. Usually, this is because the copyright notice in the comic isn't presented correctly, and that was a requirement as recently as 1989.
That's the easy stuff. If that all fails, there are two more possibilities:
4. The copyright owner has donated the book to the public domain: I don't know of any instance where this has happened, but it's possible. It's also possible that you (yes, any specific you asking) might be able to convince the current copyright holder to do that. After all, if you're really nice to them and explain that you'd like grandpa (or whoever passed the rights to them) to be deservedly well-known, they may well drop the rights or give you special permission to circulate the comic. After all, what's the average modern person going to do with the rights to a sixty-year-old comic that nobody has heard of except us?
5. The copyright currently stands, but nobody owns it: This is the other confusing part, when companies go out of business. Usually, they sell some of their assets, but overlook others. If you can trace the transfer of rights from the original copyright holder to a dead end...well, you're still technically infringing on the copyright. However, there's nobody with the authority to sue you for doing so.
Note that this last case doesn't mean "you can't find the heir." It means that you've proven that the rights were never transferred before that person or organization died or folded, and there was no contingency to do so on death (like a will).
I hope that helps, some. Oh, and remember that this is all on an issue-by-issue basis. Just because I couldn't find a renewal for Black Terror #1, I can't assume that Black Terror #2 wasn't renewed either. Likewise, even though I found Batman #14, Batman #13 might've been overlooked by the DC legal team.