Figuring out whether a work is in public domain can be quite complex as different countries have different rules and there are often a lot of (relatively) complex exceptions. There must however be a cut off point before which you can be sure beyond any reasonably doubt that the work is in public domain. Which year would this be? Or put differently: What is the oldest work which is not yet in the public domain?
And just for the record, I am aware that this might be a bit of a complex question, but it seems to be quite a reasonable question, especially if you would wish to use the work for some product that you wish to distribute internationally (e.g. this question made me think of it). Now, I do realize that other issues can come up (trademarks, later editions and moral rights for example), but to keep the question reasonable just ignore those (though as an aside they are fine).