Obviously you're unlikely to get any will-hold-up-in-court kind of evidence, but if to the best of your knowledge, it is orphaned, please say so in your post. And given that a bit of googling can usually yield an informative wikipedia entry at the very least, it would be especially nice to include a link to that as well. It's one thing to know, but it's even better to be able to tell us how you know.
And I do have to emphasize that it's important to indicate in a post whether or not to the best of one's knowledge what is being posted is an orphaned work. If I were to post a 70-year-old Canadian comic that's considered orphaned, I might know it's orphaned, but the other mods might not know anything about it, and other members reading about it might not either.
As with Warren, just because it sat around doesn't mean somebody won't snatch it up again ?
Right, but I think what we want to focus on is whether or not somebody has, and if it's known that the rights are owned by someone/a current company and/or if there are plans to republish reprints or new material.
For instance, we looked up Charlton comics, and while I had already known that DC had the rights to some of their characters, it turns out the rest are owned as well, and some of them may be heading for reprints too.
If there's no evidence or information available indicating that it is no longer an orphaned work, then you're in the clear unless new information crops up or there's a complaint.